I fly around in a cruiser and I find my tractor beams very usefull against faster moving opponents.
Either keeping them from turning the foreword arc to me in the case of escorts and the like, or as was said before keeping someone from zipping away to get healed, I tend to use it offensively a lot though, as an opponents shield get weak, hitting scramble sensors, tractor beam, evasive manuevers to turn quickly into that shield, HYT and Overload beams and just pound into that weakened shield and hull. Different uses at different times, but I like it.
Just my 2 EC's
Tractor beam does many things... least of which is to help you work on 1 shield facing...
It's key uses are:
1) Activate it on a target behind you... makes it 10x easier for Cruisers and Sci Vessels to turn around on the target and bear down with forward weapons... without tractor beam, you're stuck in a turning struggle with the ship.. which can be really slow if the ship is more maneuverable than you.
2) Keep escort class ships from bearing their forward guns on you... an escort that isn't pointing it's forward weapons at you is laughably useless in terms of threat... Escort pointing at you? Activate tractor beam... move to the side.. problem resolved.
3) Use it to keep cruisers from broadsiding you with all their beam banks... get either infront.. or ideally behind... the cruiser, activate tractor beam and sit comfy knowing that he can no longer turn to point all his broadside phasers at you... you only have to face half his weaponry while tractor beam is on.
Tractor Beam is among the most powerful weapons you can load against another ship, particularly if you're playing with allies. The problem is that it's most useful for Cruiser pilots, but almost no Cruiser pilot will be able to sacrifice Hazard Emitters or Science Team to pick it up. The best way to use it is to load it on a Science Vessel and have it support an Escort or two against a specific target.
It is very good, but it scales with AUX and so only Sci Vessels can really use it properly - and of course Sci Vessels typically don't have enough weapon damage to take advantage of it by themselves. A Sci Vessel that works with an Escort or Cruiser, though, is a terrible adversary. All it takes is Tractor Beam + SNB to virtually guarantee a kill.