Whats a healer going to do if he goes off by himself? He can't take command posts, he can't blow up routes, he can't gather intel from downed enemies, what's he gonna do? In order for a medic to be effective at all he has to be around teammates. An operative can go around BY HIMSELF and grab command posts or intel. A soldier can blow up objective BY HIMSELF.
One of the vids the devs mention that a medic can go off BY HIMSELF and wait by an objective that's is going to be used/done soon. Even if there is no one around you still get exp for just being ready to help when your team comes.