Operatives are the class for people that want to disrupt enemy organization and go for a lot of kills.
Engineers are the class for players that want to help win the mission without being directly on the front lines.
Yes, there is a lot for engineers to do in this game - that's the whole point of the class. As an engi you're always running around upgrading command posts, building barricades, constructing mg nests, disarming bombs, removing hackboxes, repairing objectives, and buffing allies - you dont fight directly as often as the other classes do, but you still feel useful without the need to go farm a lot of kills or deathmatch people in the middle of nowhere.
Personally I like that setup - Brink gives every kind of player a class that fits how they want to play, without anyone being useless to the team.
I do find operatives a little less fun to play simply because they cant buff allies, though. It would be nice if operatives could give some sort of support buff to their teammates - just as medics can improve health, assaults can improve ammo, and engineers can improve armor and damage. What if operatives could refill supply? Take the transfer supplies ability away from medics, replace it with something else, and allow operatives to transfer 1 supply pip at a time to an ally, or boost an ally's overall supply max?