'Necessary'? For what? There were blown calls before replay and baseball was fine. So was football and basketball. The only sport where replay has worked is tennis, where it doesn't slow up the action and it prevents a lot of b.s. arguments. I hate it. ANd I hate particularly the way it is used to micromanage plays: e.g., the worst case, when a player slides into a base and as he bumps over it, momentarily loses contact while still over the base. As Bob Costas noted, that play has been called safe for over a 100 years, and it's absurd to change it now. As for 'getting it right', who cares? The plays it is used to 'investigate' are bang/bang plays--essentially, coin flips. No one draws up a bunt to beat it out by a nano-second; no WR draws up his catch to tap both feet 1 mm inside the sideline. If you're trying to determine who the better team is, or who plays better that day, those calls are purely matters of chance, and all replay does is delay things. Alas, now that betting is becoming legal everywhere, I expect we'll just get more of it.