Recon's should spot and target (for the engineer or support) from far away. It seems like no ones does however. If you are going to be far away you probably need a sniper rifle or DMR. You can use the PLD, SoFLAM, MAV, T-UGS and SUAV to do that. Then your supports can use their mortars to rain death on infantry. You engineer can use one of the launchers that is guided to a painted targer and you can snipe from a safe distance. Most recons however just seem to sit on a hill trying to get headshots for their You Tube channel.
I don't think you would have medics playing the game if you let everyone equip and ARs. Revives don't show up in that final screen but you number of kills do.
Carbines don't quite have the range or accuracy the ARs do so I suspect it is a good compromise to give them to everyone.
BF3 allowed PWDs for everyone and I use to use the hell out of the UMP-45, the ACR-W or ACE 52 seem to be good alternatives to that.