Well what do they do? They don't seem to do anything apart from time-keep and make managers stay behind the tecnical area.
Your going off on a tangent mate anyway with this ref thing (they are apart of football as anybody, but that is another topic). I just personally don't believe in technology for footy.
An example of a major change in football over the last 20-30 years for instance - the backpass rule. Ok it was a major change, it changed the game, but it was the same rule for the top down to the bottom. Video technology being brought in will only apply to the top clubs. Say it is brought into the Premier League, it won't be brought into the lower leagues, because they won't be able to afford it/maintain it. So with that in mind Man Utd draw Accy Stanley away in the FA Cup and Accy don't have the equipment, and Rooney scores a 'goal' but it is disallowed because the lino couldn't see if it crossed the line or not....
SAF then comes out and says; "In the Premier League, that would have been given, but becuase we're here it hasn't"
So not only do you alienate the lower leagues and create a bigger gap between the Prem and the football league, you take a part of the game away from the masses of people who play every week.
Hypothetical i know, but if it is brought in, these are things you have to consider might happen, not just a case of 'was it over the line or not'