This is gonna be football orientated, sorry. As far as cricket goes, I support the new system if it means more decisions are made correctly. I dont agree with "i see no definitive reason why i shouldnt support your decision" bollocks. Man behind the camera should make his own umpiring call based on what he sees. If he REALLY REALLY REALLY cant make up his mind then a) the on field umpires call should stand and b) the umpire who can see multiple replays shouldnt be an umpire.
On to football, more specifically, responding to Tick.
First of all, American football is stop start and so stopping the game just happens when there is a break in play. In football this happens a lot less regularly and so such calls will break up the game.
Secondly. the calls can be so difficult that video technology might not help in some cases. Take Ronaldo's "penalty" incident in the Carling Cup final. The ref saw it as a dive in real time, I saw it as not a dive but not a penalty, Fergie's eagle eyes probably saw Ronaldo suffer GBH. The pundits in the studio (though morons) all say it was a 100% penalty and they had the benefit of video replays. I'm sure, nonetheless, that people would say with the benefit of the cameras that it was still a dive, that he was on his way down (which it does look like).
In such a case, we still have so much subjectivity and so which decision can be made?
I agree it might be a good idea to let managers have 1 or 2 shouts per game (not with the points deduction system and at least it will have to be more than 5), but I cant see past a) the breaks in play and b) the fact that some decisions through a camera will be as hard as to the naked eye. Sometimes, the man on the pitch just has a better view and a better feel in that moment than a man behind a screen.