Bombs, Grenades, Flamethrower kits all become switch keys (where the hotkey is), so you can switch back to pistols or a rifle, but the Med Kit, doesn’t become a switch key. It just keeps enabling the kit even if you are already in it. Try it out.
ArenaNet should make the Med Kit a switch key when you are using it, like the other kits.
I’d also like the tilde (switch key) to switch back to the last kit you used. Whether that’s Med Kit, Flamethrower, Bomb, Grenades, etc.