In the most recent case of "MLB Needs To Use Instant Replay" the Dodgers and the Rockies wasted six minutes of everyone's life by arguing with Colorado's manager Jim Tracy, and then consequently ejecting him over whether Dexter Fowler caught a Shane Victorino hit, or if the ball bounced.
Even with replay, and watching it in slow motion, it's tough to tell if he absolutely caught it or not, which means without instant replay, it's nearly impossible.
In the video, Vin Scully expertly re-dubs Tracy's "blinkin' fertilizer" to keep the broadcast family friendly, and explains why there should be instant replay:
There's no way that instant replay would slow the game down more, and players and fans would feel more confident that they had seen a fair game. Sure, even with instant replay refs make mistakes, but they much less egregious than the ones umps make, and they take a lot less time to make them.
"We have all this technology and they don't use it because they say it would delay the game. Well, what was that we just saw?"