I seem to remember schema talking about letting us use logic to switch reactors at one point. It MAY have been replaced with the "auto switch reactor" chamber that switches to a fresh reactor when your current one overheats.
Multiple reactors is more of a Combat-tanking thing(in theory), but it does save you a bunch of clicks if you want to ave multiple modes.
Easiest way is to remember you have the Insert-key as default reactor hotkey.