Every build is viable since there are no enrage timers to encounters. Yes you can clear every dungeon with most builds. But no, I would not want you for my "fastest, speed clear" runs. That doesn't mean I don't like you, just that your setup is not efficient "for that purpose." If i wanted to just get the dungeon done regardless of how fast it was, then yeah I'd definitely hit you up for a party. This thread was posted in regards on how to be Optimal for Dungeon clearing as fast as possible with the best builds to do just that. There will always be a fastest way of doing something, not everyone has to do it that way.
I just reread the original post and don't see where the OP specified s/he was speaking specifically about speed runs. What I took from it was that if you don't spec for max dps, you're just wasting other peoples time because your build is suboptimal. Why do a run in 10 minutes when you can put out more dps and do it in 6? It just seemed to me the OP believes warriors are a dps only class and anyone that goes in another direction is suffering from a "loser" mentality. Perhaps my reading comprehension has diminished with my advanced age! ;-)