Of course you have to draw the line somewhere. They drew a line in the NFL and I don't see why they couldn't draw the line in baseball. I don't think anybody is calling for a review of every play. Maybe one coaches challenge during the regular season and two during the playoffs for certain calls only.
For instance, in last night's Philly/Colorado game 3, at a crucial point late in the game, the ball hit of Chase Utley's bat, bounced off Utley's leg, and trickled into fair territory. I think the correct call is a foul ball. Utley proceeded to first and was ruled safe in a bang bang play. Although I don't think the play at first should be challengeable, I think what happen at the plate should be.