Fortunately there are a lot of MMOs where glass cannons are non-viable or inferior, because of the lack of a "trinity" of classes (heck, its probably the majority of them if you take out direct EQ and WoW clones). SWTOR is unfortunately not one of those, but stuff really toss around a lot of damage that ignore the people on top of the threat list.
The final boss of EC, for example, really only has one difficult-ish part (not really, but...). A point where 2 droids go around shooting people for 15k damage, and they can't be taunted. While we managed the fight just fine, the people in the group who had switched out their mods to be pure glass cannons were bordering on being liabilities. A lot of other mechanics in EC will make lower hp people have to dodge a lot more than they should, no matter how good the healers are (there's always the GCD to deal with...). Hp isn't everything, but a large amount of endurance for a little bit of offensive stats is probably a desirable tradeoff for many classes, especially melees.
As another note, in EC, mechanics are everything. Its very easy to "miss" a mechanic that, if handled properly, would let you put out way more damage. This is most visible in hardmode, but its there in story mode too. When we finished story mode we had people with orange gear with 1.1 daily mods in it, and had no enrage issues on our successful runs. We only had a single ranged (having more would make it even easier). A columi group can totally do it on Story if done the same way a Rekata group would do it on hard. Obviously full rekata/blackhole will make it a lot easier
Well, hell, this finally explains why I'm getting slapped around so much then. Right now I'm one of the orange gear guys, sitting on about 1500 Aim and less than 15k hp. Guess I need to work on that. Thanks!