Instant replay and challenge systems allow the teams to have better calls by the officials and to reverse calls that are incorrect. While they can slow down a game a little the help to insure that the outcome of the game is fair for both sides and that the score was achieved is correct.
Instant replay in football would not be traditional if it was implemented on a large scale. This is a controversial subject so there will be some supporters as well. However I think that in the end it is much more effective to keep American Football the same and not include any additional instant replay tools.