While I don't disagree at all with the logic of the above, speaking as one who is very often one of those ranged toons who ends up playing free safety when the tanks (and not always the tanks these days) run off to the next group while I clean up their mess, I can tell you I don't view that action as "confidence" in my ability. It looks and feels a lot more like, "I'm going to take care of me, and the hell with you." Fair enough, save when you bail out and leave multiple purps and oranges, which me and maybe 2 other people have to clean up, thus putting us behind the rest of the group while we sweep up the detritus.
And while I solidly agree with the argument that I'm in charge of my own fun, and therefore don't have to play with bumrush teams if I don't dig that, it is increasingly the standard mode of play, which, for some of us, is something to be more tolerated than enjoyed. I'm not gonna begrudge anybody their steamrolling. but, honestly, teammates don't seem to need each other near as much as they used to (except, as this example attests, to clean up their scraps). We've gained a great deal, but this feels like an unfortunate byproduct of making the rare run-of-the-mill.