Any element of stealth/surprise will be completely cut out of the game, theres more negative feedback on bf3 having this as well, considering how bad of an experience it people had with it in BC2. Id honestly say the best working method of spotting would be that of Battlefield 2's. Not only did one have to spot an enemy, having to use the comm rose, but it also made the minimap an extremely useful tool in the HUD. It also encourages people to communicate with one another a bit more than just simple on screen spotting that made BC2 squads so useless.
If we were to exercise the 3d spotting we will all notice how simple and arcade this mod will end up developing to.
BUT... If this poll end ups being too close, I could see 3d spotting being added as a reward for SQUAD POINTS so it will encourage teamwork and reward for it as well.
For example, lets say 3d spotting isnt worked by a player but rather its a UAV thats ONLY FOR THAT SQUAD. This way a squad will be rewarded and encouraged to do teamwork (Revive, Individual spotting like bf2, Resupply, etc..) and only the squad leader will be able to activate it. The UAV will spot every target on the map but will only 3d spot onscreen enemies inside a radius of lets say 50 meter of a player (so ones wont be filled with little 3d icons if there were to be a full server). After the UAV has been used it will have a recharge time thats long enough so it cant be abused.
Im probably going to go off topic on this one but if this integrated into Casus Belli, a reward system would probably grow out of it. But it'd be better if nothing more helpful than little things like UAV, supply box, or a rally point would be part of the reward system. Nothing too rewarding so people wont be only concentrating on getting the awards.
There... more than just my 2 cents