telecaster pickups, P90s and humbuckers already have baseplates fitted. strats single coils don't.
i'm no expert, but the baseplate seems to give body and meat to the tone, especially in the bridge, whilst remaining typically single coil
i've a slowhand bridge with a baseplate and it sounds "well meaty" (quote from Will) - theres a clip in the forum, though i haven't really captured the 'in the room' sound very well. it sounds like a sort of cross between a P90 and a strat i think.
so i say 'yes' to baseplates - though i don't know if they'd be needed on something like a sinner or trilogy.
maybe you can retrofit them - they seem just to be screwed on. don't know for sure - ask tim about that.