First: You only need to keep the list on the server. As I said, only the light state needs to be replicated.
Second: players go into/out of areas quite infreqently (compared to “firing weapons” or “general movement,”) so even if you did replicate this list (which you don’t need to,) it probably wouldn’t be a big deal.
The draw-back of a counter is that if you ever have any bug, the counter is off forever. The list (or set) will self-correct if you accidentally call “add” or “release” more than once for the same player.