I believe the reason a lot of these major league stars skip these kind of things is because the major league season is already long enough. Between Spring training, the regular season, & the playoffs, any given player could end up playing near 200 baseball games from early March to early October. There is already a great workload on them.
Well, the season begins the beginning of March, which is when the thick of Spring Training begins. As a team owner, I would be concerned about the possibility of my players getting injured.
As a player, I think I would love the competition, which would prove to be a much more effective (and exciting) spring training for me...