Since the changes to how weapon masteries are applied, you only receive a physical power bonus for weapons you are actively using. Here's an example of some weapon stats when using different weapon combinations. These were tested on a cornac temporal warden with 40 str/dex.
* With weapon/knife/exotic masteries at [1/5]
Main Off MDam ODam PhysP
Sword Dagger 36 15 35
Sword Whip 36 18 35
Dagger Dagger 29 15 35
Dagger Whip 29 18 35
Whip Dagger 36 15 35
Whip Whip 36 18 35
* With only weapon mastery at [1/5]
Main Off MDam ODam PhysP
Sword Dagger 36 11 35
Sword Whip 36 13 35
Dagger Dagger 22 11 30
Dagger Whip 22 13 30
Whip Dagger 26 11 30
Whip Whip 26 13 30
While the each mastery has its respective %dam bonus applied to appropriate weapons, the physical power bonus is only added once. So, it is most effective to specialize in one type of weapon. Temporal wardens are particularly affected, since they have weapon sets which have mismatched weapons.
13:51 < Supermini> right now fighting with 2 daggers costs you 5 points in
masteries, fighting with 2 one hand weapons costs you 5
points, fighting with 1 of each costs you 10 points and the
second 5 invested only gives you damage% and not the
physical power boost. That's the essence of it.
...
13:53 < johnny0> supermini: exactly, it narrows down effective choices and
essentially doubles diminishing returns
13:53 < tiger_eye> so, yeah, what Supermini said
13:53 < johnny0> since physical power is already subjected to diminishing
returns
...
13:58 < tiger_eye> I wouldn't be against having a boost for wielding a sword
and dagger if one invested in both sword and dagger mastery
13:59 < Supermini> yeah
13:59 < tiger_eye> power = power + mainhand_power + (offhand and offhand_type
~= mainhand_type and offhand_power or 0)
14:00 < tiger_eye> for those who speak pseudo-lua ;)
...
14:06 < tiger_eye> requiring them to be different does actually provide an
incentive for, say, investing in both weapon and dagger
masteries, which I think is fine
14:06 < Supermini> the issue is with mismatched weapons, that cost you 10
generic points instead of regular 5, and give you less
14:07 < tiger_eye> exactly, gotta spend those precious points. that nearly 25%
of the generics!
Just have it use the higher of the two applicable skills and be done with it, instead of averaging them. That is, apply the physical power bonus/weapon multiplier of the highest applicable skill to dualwielding two different types of weapon.
For Weapon Mastery vs Dagger, dedicated weapon and dagger builds are both going to exist in exclusive states, due to skills requiring certain weapons and due to the strong Dex/Cun orientation of two of the Rogue classes. For Sword/Dagger, anything else makes it cost more, and given the statistic differences involved, it mostly exists to nerf the two classes that can realistically utilize that combo well(Marauder and Cursed...maybe AB too, though.), and the combination is not enough higher power to justify extra Generic power or deep thought about it.
The only other time this would come up involves Exotic Weapons Mastery, and I can't see anything wrong with rewarding dedicated Exotic Weapon building. Even with strong buffs to them so far, the only people that consider it are probably those getting Legacy of the Naloren, and that has strong Trident orientation.