In any case, to sum up what I've seen in this thread:
Stairscumming by itself is:
not interactive
awkward for positioning, aka getting surrounded or luring enemies to the stairs, waste of time and effort
its boring
inefficient, obviously if you have to use a plan c strategy to win, you did something wrong

What its good for:
characters with a difficult start (cursed)
allowing the legitimate possibility of 'challenge' characters who have an exceptionally difficult start (yeek zerker woo!)
new players who lack in-game knowledge of other tactics
does not severely punish new players who make mistakes (im sure there are those who tried attacking The Mouth, used the stairs to escape, thought for a moment, and go kill the real targets)
certain encounters/achievements impossible with stairscumming, forces players who relied on it previously to try new things!
stairscumming is boring so most people try to figure out quicker/better ways to play
Bad stuff:
pisses off hardcore players, those noobs should play the real way!
some players consider any scumming "cheating" and "gives an unfair and unneeded advantage" <-- this line is about perception, does not reflect my opinion or the statistical truth, blah blah
its a pain to design layouts/write code to eliminate stairscumming
the simplest method to stop stairscumming leads to insta-gibs, aka the 1 turn method, which are undesired by noobs and hardcore players alike
there will be those players who will abuse this, just like any other game mechanic, regardless of how boring or how little benefit it actually gives compared to intelligent play or build or items