Many things factor into people not being a support merc, one of those factors is mission objectives. Now personally I like to play Rhino, but when I can I’ll help pick up my teammates or work on an objective in between mowing down the enemy, it’s slower than support but still helps everyone out. One thing you need to consider is your role, yes you can perform every task the gamemode has as any merc, but some are better suited for it, so while slowly repairing an objective is undesirable, remember that you should not focus on that. You can always repair and revive, but don’t treat it like it’s YOUR job because that’s how you lose, you can always step away and attack the enemy if it’s the better option because your support abilities are weak.
For instance, as a Rhino reviving a teammate I will try my best to get them up, IF an enemy begins attacking me during this, I will stop and kill them, yes this isn’t desired for my teammate but both of us ending up dead is worse. If I am repairing an EV slowly and someone attacks, I will of course stop and fight back because in the time it takes them to kill me, the progress will have not gone far as it would for an engineer.
So with that said, play your favourite mercs, Skyhammer will certainly help out your team in other ways(Everyone always needs ammo!) and Phantom could allow you to sneak behind and really cripple the enemy team. Just because your support abilities are slower doesn’t mean you don’t have to do them, but in the case that you do, you do not have to commit to the action.