In control and sabotage, a bubble can turn the tide of an entire game. If someone is lucky enough to have a smash or void bomb available, yes it will break. If not, nothing else can stop it if you have more than one person in it.
Bladedancer charges in? Laugh, take the first hit with your overshield and blast him in the face with the shotgun twice. Golden gun? Yeah, doesn't work. Sunsinger? A single grenade won't kill someone with an overshield, and the oversheild regens if you just walk outside the bubble and go back in. It basically gives you a point on the map that's uncontestable unless you have a void bomb or a smash available. Not to mention you can spec it so that it gives off orbs of light en masse when it takes damage.
Outside of control/sabtoage? It's useless.