But as long as they serve different functions, you don't have just one "best" charm that you're going to use whenever your charms come off of cooldown. If you need to escape, your cure poisons charm does nothing for you. If you need to cure poisons, your thermal shield charm does nothing for you. It doesn't matter how good your cure poisons charm is, it's not going to help you escape. Similarly, it doesn't matter how good your thermal shield charm is, it's not going to help you with that nasty crippling poison. Thus, multiple charms have the potential to be useful. I'm not saying they always will be-- that depends on the implementation. But as long as each charm has a unique, specialized function, the shared cooldown will not make it so that you only have one useful charm.
Theoretically yes, but not true in practice, for a couple of reasons.
Firstly, there's the general nature of Charms. If you are going to cure poisons, using a dedicated Physical Wild is more optimal; You only have one universal cooldown, however said Wild will run on a different cooldown(obviously this depends on if the enemy has a good Stun or not, but in general). If you have 70% fire resist, your Thermal Shield is not going to be particularly important.
If you have ways to deal with these issues in other ways, you should use them, because they run on separate cooldowns(or, potentially, not cooldowns at all, in the case of some skills such as the shields or thorny skin). Yes, they're nice in an extreme case; I'm pretty sure I'm going to be lugging around a few dozen Charms for the theoretical extreme of getting a really nasty Fire/Wildfire Elite, or just in case I get pinned between a Ghoulking and an Armored Skeleton Warrior. You don't survive reliably in a roguelike without planning for the very worst case.
But having said that, by and large, you're going to use whatever you find key to your class, which cannot be reproduced in other areas. If you're, say, an Anorithil, that's going to perhaps be wands. In general, it's going to be healing or teleportation. Especially teleportation. Psychoportation works through Antimagic, works through confusion(Cute trick with itemcasts, and why you should love Serpent Cloak), and is an extremely powerful(though not entirely reliable) escape that not a lot of classes get naturally.
You don't want your teleports to fail if you can help it. You don't want your teleport on cooldown if you suddenly decide you need it. An extreme situation escape is much weaker if you can't rely on it as much as possible.
Of course, as it stands, all the charms in general have some niche for someone, but here comes the other half of the issue.
You can't always create charms for a given class, that do have a niche. If you have passive and sustained charms, there comes a question of how cooldowns interact with them. If using an active charm sets your passive or sustain on cooldown, it becomes constantly unreliable as support, and lowers the usefulness of those aforementioned odd niche items even further; Now you have to decide if curing that disease is worth at temporary loss in saves(for a Chant of Fortitude charm, say), which, if you're in a combat situation still, may just get you diseased again instantly, for example. It's an addition in complexity with a lot of work behind it, for diminishing returns, as all of this is still fundamentally fighting over one cooldown.
If you don't create those, of course, it becomes nearly instantly unworkable. What do you make for an active skill Fighter charm that won't be less useful than, say, Mind Sear, for most characters? (Especially now that Mind Sear's going the easier to heal, less common to consume, Psi route instead of Equi?) You could, of course, retool the entire Escort set to fit this, concievably, but it's going to end up ruling out nearly every physical fighter.
The extra 4 to physical and spell saves from one extra Sun Paladin is really not that big of a deal.
Actually that varies a lot. I seem to recall Chant of Fortitude runs off a standard spell equation, so you get about a 40% boost from L1 to L2, and another 30%(on the base, not on the L2) or so for the L3. This ends up being a lot more than 4 if you're a dedicated mage, though not so much if you're a Bulwark whose only real source of Spellpower is lugging a specific fixedart lamp around. Of course...
Maybe I'm just bitter that I got six Sun Paladin escorts on my last character, and no Seers or Anorithils. But these kinds of things do happen in the current system.
...if you get stuck with having to raise Chant of Fortitude from L4 to L5, yeah, you're not likely to see much more than 4 points on anyone. So yeah, that's always sad. Of course, a limit on a specific type of Escort would, honestly, be a welcome change; Getting five Anorithils and hitting L5 Providence is ridiculous, getting five Warriors and hitting L5 Unflinching is silly, yet getting five Seers and hitting L5 on Premonition is...yeah. Not very useful, or fun.
(Also, I feel your pain. My poor, poor Paradox Mage never saw an Anorithil. Not that she really needed it, but I wanted the hilarity of a mail wearing tank healer Paradox Mage.)
I think in theory the permanent stat bonuses are there to counterbalance that possibility, but...well. While the stat bonuses may even outweigh a skill in many cases, they never are as much fun as a skill, so few people seem to take them. I know I've passed up times where a stat struck me as the objective best answer, solely because it was also the most boring answer.
I'm not going to say that Escorts are the only answer to the flexible skill ideals. Perhaps it would in fact be better to make skillsets more reliably available; Move current Escort skillsets to tutors throughout the world, who grant you one of the skills they normally can or their skillset, then make common Escorts give charms.
I could see that, and it actually has an interesting rebalance effect, potentially; Loremasters as an early option when their abilities are most useful, Anorithils and Sun Paladins-both of which have strong effects-as a set of Far East tutors, where they stand out less and require lategame retools of build for their categories, something the game basically does not ask of you as it stands, that sort of thing. Adding a bunch of Generic categories flat rate is also an interesting idea.
I did personally agree with your last point, however.
donkatsu wrote:
However, I'm not sure this overhaul is worth the time to implement, either. There are much simpler solutions to fixing escort quests.
Ah, yeah, sorry about that.

Missed that particular line.
As to b39 challenge...mmm, I think that, at least for my tastes (which may be on the higher side of challenge; I got into Roguelikes playing Dungeon Crawl, and only gave it up only when they stopped adding meaningfully to the game with newer versions, IMO.) the game may need some little tweaks upward on challenge, now that the random god-elite with that can hit a player every time, period, and possess 140 Armor with 100% armor hardiness(Or 85 Defense, your preference. I was scared to *death* of running into one of those with the Cursed...) can't happen(not that I've tried to compile b39 yet; I just follow the tracker because it's fun. So that's mostly going on b38 plus knowledge of b39's changes.).
But if DarkGod implements randomized Elites as a general thing, which I seem to recall being in the planned ideas section somewhere, that challenge issue may be fixed quite rapidly(How would you like to turn a corner while tactically retreating from a known, fixed position elite, only to run into a randomly placed elite Orc Pyromancer that can Fearscape you? Sounds interestingly hard to me.). Quantity of varied elites seems more interesting than quality, honestly. I wouldn't worry about it a lot.