In overland questing and normal Dungeons you have flexibility to do content without an optimized group.
In harder content such as Veteran Dungeons, you're not doing it without a tank and heals. (You can however run with a healing tank and 3 DPS, etc) and you have a lot of versatile builds with which you can accomplish this.
When you move to Veteran Trials, you do have to have a much more optimized group and there are builds which excel beyond most make it easier to succeed.
Your ability to play different roles on any class however do make the game more inclusive and the diversity of builds isn't matched by any other MMO at the moment in my opinion. This however does allow for bad builds that just don't work well, but I prefer the freedom to find that out myself.