I've never used the mechanic except for fun when I come into a half-finished game. I can tell you the doctor is an extremely fun class though. If you can learn to use tanks and always align yourself strategically in a conflict, you're basically immortal and give all your squads and fleets a HUGE tactical advantage. Coming in top three is almost guaranteed.

Basically, STAY BEHIND PEOPLE. The longer you are alive, the longer they stay alive. Keep your fire consistent, but realign yourself constantly to stay away from exposed flanks and tanks that are going to die. Try to predict which tanks are dying, realign, etc. Its much more important that you survive to heal your enemies next target than it is to try to do them damage. Don't look at your heal timer during a large conflict, just try to right-click every time you realign which should be every 5-10 seconds. When a conflict is winding down, get a good defensive position the max distance away from your enemy you can while still holding fire without advancing, and save your last heal to escape. If you time this right, your squad/fleet dies, you rush back to base, they all respawn, you speed-burst them back to enemy base, and and the way you guys pick off the few stragglers who tried to pursue you after you fled. HEAL ANYONE YOU CAN when you're not in a squad or just wandering around. Every time your teammates die, the enemy gets scrap. Also try to give speed bursts to anyone struggling in a conflict that might want to run. In close game you can use the Doctor to Kamikaze the enemy base. Either go in with a small squad and get as close as you can to the enemy base, sail in, do some damage, then speed burst out and repeat. Or just speed burst from respawn, get close, wait for next burst, burst in, do damage, die, repeat. Or whatever, you get the idea.

Doctor can be a lot of fun and played right will almost always end up in the top 3. Good luck!