DPS is the job of DPS.

[Shurrikhan]

...and before someone says something like "those other games made tanks/heal jobs more integral". That might be true, but at some point as people get stronger, everything becomes a little less necessary. I remember having to DPS as heals while doing ICC10/25 in WoW as we all got stronger, and even one boss where I could DPS the entire time while the other healer would heal every 10 seconds and DPS the rest of the time, and no one made a stink either way (even though literally everyone had parser addons), same with choosing between 2 tanks that could both hold aggro and both knew the fights and picking the one that had the higher dps and telling the other one he could only join on his DPS spec because his tank spec didn't do enough deeps - answer was usually "ok, one sec".

I think all that might just have something to do with a lesser degree of role fixation, or certainly within the experience of most players playing during that time. Most "who's the best equipped to..." within any particular global was usually a trick question in terms of role.
Nominal physical incoming damage at low expertise: who should take it?
The melee DPS over there!
What, why?
Because he's Arms.
So?
Massive counterattacks.
More than the tank?
Hugely.

Heck, you still had some older style talents that outright encouraged DPS to swap in and take some damage, or tanks to occasionally heal, etc.; they were still primarily just "toolkits", free from oversimplication.

Plus, PvP simply took off both more highly and more openly there, and when strategies aren't reducible to tank-and-spank, resource-exchange, where a bit of DPS here saves required healing there or a bit of tanking here costs additional healing on the enemies' part, is a given. They weren't told "this is your box; squat in it," by the design or the community, so even in progression raids there was just one rule -- do whatever's most effective, given the current and upcoming condition of the group.