Yeah i'm serious, that extra bit of health has saved me a number of times. Why would I need decoy when I can cloak to remove aggro or use combat drone to distract an enemey e.g. Phantoms. Combat drone is a much better decoy anyway
Sorry, I'm going to have to try to convert you. 
Combat Drone is better but it's not much better - they're effectively the same power, just with different range capabilities. Asking why you would need a decoy when you have all those other things is like asking why you need gravy on your steak - you don't, but it's a whole lot better with it. Fitness points are generally a placebo above Silver, and are particularly so on the Volus characters. I would honestly struggle to imagine or believe in a scenario on higher levels where 825 shields has any practical benefit over 500. The guy is already almost immortal, after all.
A Geth Prime doesn't need a turret and a drone, but he has them both, and a Volus Merc is one of the greatest characters for getting genuine payback on those cheap cheating scumbags. Two staggering powers and a stagger-capable weapon (if not using a Saber for style) with stagger-capable ammo is more fun than most other options in this game. Yo Dawg, yeah, I do like some stagger, especially against Geth. I'll admit that he does a little less well against the other factions, but damage output is always better than notional or imaginary survivability.
Imagine turning up at a Volus Merc Party. It needs 8 little guys for it to feel right. Do you really want to be the one there who doesn't have a +1 with him? Awkward....