Well, I always play:
1.Fire Support
2.Engineer
3.Medic
Simply because that’s what I’m good at. Sometimes I’ll bring a sniper if I’m in that mood. Fire supports are often useful outside of EV maps, for clearing out areas, and, y’know, calling in supporting fire. Explosions from the sky are often useful, even when you don’t kill any one. You’ll say “But they can just move, and avoid the explosion.” Well, yeah. They moved. They’re gone for a second. That creates an opening in the crossfire for your slayers to move up.
This is an excellent reason why you have squads. If you could play any given merc at any given time, you could just cycle between Skyhammer, Arty, Kira, and Stoker, thereby circumventing cooldown. Same with medics. K for suicide to switch between the best suited medic for that moment. The same an apply to any class. If you aren’t forced to play the merc you brought along, you’ll never learn versatility with any of them. You’ll just play one merc in one situation and then switch to another. Yes, some mercs are better at some things than others, but when you have to make do, it creates a level of improvisation that is a lot more fun than just doing the same thing with the same merc over and over again.