It has nothing to do with trinity. And it actually is exactly how people in CoH play.
Some ATs can fill multiple roles, some can't (or effectively can't).
Scrappers, Stalkers, Blasters, and Sentinels can only DPS. That is their role. They can help out here and there with a little aggro management, or a little debuffing, or a little control, but effectively they have one role.
Tankers, Brutes, Defenders, Controllers, Dominators, and all the rest all can fill at LEAST two roles, sometimes more.