Realism vs. Gameplay
Well, again, I think this all becomes an issue of realism vs. Gameplay. I personally agree that it should simply be up to the Server Operator to set whether the Radar should be on or not. I also think they should be able to set the Radar to only show allies or to show both enemies and allies (and maybe only enemies). An option that will only give it to one team would be cool too.
There are many reasons for this. First of all, it's not completely true to say that Radars are completely unrealistic. The US government has had technology using Satellites to give updates on where people are located for years. Proper prepartion with cameras and such make it possible as well. Sure to the infantry unit, this will not appear on a Radar but they may know in general where hostile forces are located because of these technologies. The representation for this may simply be in the radar.
Second, it's really dependent on the map whether a radar would end up messing up gameplay or improving it. For maps with lots of open areas for instance, snipers would be too good if there were no radars to give you some idea of where to point that grenade launcher at. In maps with nothing but small confined areas, however, it could easily ruin the game, encouraging people to chuck grenades into the small rooms or corridors when he sees a group of little dots in his radar.
Thirdly, having flexible options for Radar will allow map makers to set-up interesting scenarios such as, one team has a strategic advantage with nice cover, etc., but the other team has better information base (in the form of being able to see where the opposing team is on their radar).
Fourthly, the issue of whether a radar will improve gameplay or deteriorate it will also depend on the type of players that frequents the servers. If half of them are campers and alot of the players tend to go idle, it would be very frustrating if there were no radars as people end up playing a waiting game quite often. If, however, most people are considerate enough to look for each other to minimize the amount of time people spend waiting when there are only a few players left, Radars will not be necessary.
Here's something on my personal wish list though. The radars should be adjustable on the server to show enemy units for 1 second only every set number of seconds, after which the radar will go blank until the next 1 second. So for instance, the Server might be able to set it so that after 10 seconds, the radar will activate, show where everybody is, then go blank. After the next 10 seconds, people will see where everybody is again.
This in my opinion would be very cool. It would make it so that it would be easy to spot campers (and idle players), but not people who are constantly moving around. By making the time adjustable, people could set it to suit certain maps.
I guess what I'm trying to say is that in terms of the Radar, it should be customizable as possible to allow the administrator to decide what is good for his server and what is not. There are advantages and disadvantages to both. However, in certain situations one may end up outweighing the other in which case being able to set the radar settings would be a God send.