The intent of the developers was to have an RPG combat system that makes healing and tanking optional at best. We could debate whether or not they foresaw the consequences of that, but I think it's obvious that if you can successfully complete dungeons with no healer, no tank, and no defensive stats, the variable you have left is how fast you can do it. While handicapping yourself can sometimes be fun to introduce a new challenge, you can't blame the players for generally wanting to take the easiest available route to their goals.
I think you're also ignoring the possibility that build optimization is a huge part of the enjoyment of this genre for a large number of people. Despite playing in a casual guild similar to yours where I could probably get away with running anything I want, I still usually choose direct DPS, and I still want to min/max it.