I think it would be agreat addition to any online league. GMs would be allowed to bring 50 players to spring training (50 is the max that the roster editor will allow). This would be a good feature when you're looking at a position battle at the beginning of the season or if you were trying to decide whether a rookie is ready for the big league roster.
Just watching spring training games this season already, I think it would be great if you could actually manage your top prospect and take a look at him each year until you think he's ready to break the club.
If anyone is interested, I'm simulating this year's spring training games on my web site with the roster set I'm creating. You can view it
HERE