However, I suppose you could argue that valuable content would be blocked to some extent, it a knowledgeable contributor was put off because s/he didn't have time for a longer comment. Still, 50 words isn't much, or 30, 25... anything above the average sentence length I suppose would start to have the desired effect.
The problem with time wasters is that it's no big deal for them, they're already wasting their time. It's always going to be up to users who care like you, me and others to try to mitigate the exposure of those time wasting comments while not penalizing legitimate contributions by users who care, and that's why I don't think a hard comment length should be implemented.