Support needs the grenade bonuses because he's the one who welded all the damn doors.
It's been already established that the firebug not having armour is the bug.
The medic's bile resistance is part of the medic's job in rushing into bad places.
You don't "really" "need" any of the perks at all, so I don't get what your "isn't something ___ really needs" attitude is about.
Damage resistance != more max hitpoints, I thought we'd gone through this. Nevermind the mathematical difference of what sort of damage it's better against, but... Let's say a medic's shot heals 40 health (pulling the number out of my bottle of glass). Right now that always means boosting someone back up by 40%. A target with 200 HP however enjoys only a 20% boost. So you either scale the syringe according to the target's health (and make each HP increase that much more potent), or leave things odd and counterintuitive, or offer an upgrade for the medic to match this that creates yet another arms race within the game development.
Right now we've got gradual increases in player firepower, player abilities, monster health, and monster attack power. Player health is the only constant metric by which we can measure the absolute power of a given actor - and if we lose that, we might as well scale everything like another 80-level MMORPG with no sense of proportion between two random human beings.
I don't think most of us bought KF to play that kind of game.