My two kids are 19 months apart, and they've shared a room since the younger one was about 6 m/o (he was still co-sleeping with me sometimes then, but grew out of that pretty much by 9 m/o).
Drawbacks: in addition to the possibility of the baby waking up the older child (which probably won't happen much -- the older child will likely get accustomed to the noise and won't wake up unless the crying is major and/or long; a fan or white noise machine will help) -- if you keep their toys all in their room, and they have different nap-times, they won't have anything to play with while their sibling is asleep. If you have a play room or lots of toys in the living room, this won't be an issue; but if you try to keep your room clear of toys, it might.
Benefits: they get used to sharing a room, so later it won't be a big struggle or ordeal to get them to share a room.