I don’t have enough data to back me up, but I’d imagine it’s not the best thing for team spirit to effectively say “You’re not good enough to play this round, Tiger [or whoever] will do it instead”.
I'm sure Tiger will play all five sessions, but then he's probably the fittest, mentally or physically, around.
Looking at this afternoon's pairings, both captains are playing all 12 players on the first day. Which I think is a good idea.