Holding the tongue is helpful for oral examination but is not a humane method for whole-horse restraint. Pulling on the tongue excessively can damage the hyoid apparatus and even lead to cranial nerve damage, so when examining the mouth one should always brace the hand holding the tongue against the bars or the cheek, so that if the horse moves there will not be excess traction on the tongue. I'm not saying your vet did anything wrong, but holding the tongue correctly does not prevent the horse from moving its head (although by bracing your hand against the bars you may gain some control of head movements), let alone its body. If your horse stood still, that says more about the vet's finesse and the horse's temperament than the restraint technique.
For other aspiring equine vets, I think alliecats' comment about SICK horse handling being different from WELL horse handling bears repeating. As discussed on another thread recently, not only can normally mild-mannered horses do dangerous things due to pain and illness, when you're a vet, a number of the horses you deal with will not be mild-mannered to begin with.
With any species, I think it's easy to get complacent after working with them for a while because of being hurried, distracted, or overconfident. Any animal has the potential to hurt you, even one who has been cooperative in the past. Some situations are higher risk than others - working under a horse, working with a large-breed dog's head close to your face, painful procedures, etc. - but the minute you stop paying attention to safety is when you put yourself at unnecessary risk. Also, as mentioned above, NO animal is worth the safety of you or the people around you, and NO client pays you enough to compensate you for disability, death, or even just a lousy week of not being able to work due to injury. If your gut tells you not to do something or not to handle a particular animal, don't do it or find a safer way to do it.