I think substitutions in baseball are just fine. You could take too much advantage of things if you could allow players back into a game. Roster limits would help curb this, but there would be nothing to stop a team from running out 3 DH type players and readily substituting in a fielder every inning. Sano could be penciled in at SS, replaced by Escobar every half inning back and forth. Or....Sano gets on base, gets pinch run for, and then re-substituted in for the next batter. Too many ways to take advantage of a baseball game by re-using players.