This one has me boggled at times as to why some riders think that the trot has to be perfect before the can canter the horse. This really seems to be the case with horses that are green. There is no reason that after a horse has had 30 days under saddle that canter can't be done. This is not to say that the horse is to be cantered for the entire ride or for long stretches. It should be added into the workout so that the balance will improve.
My philosophy has been the canter improves the trot. You can trot all you want but it won't improve the quality of the canter. This is why after the horse has had a canter the trot is more forward and rounder.
The canter is more natural to the horse than the trot. When horses are turned out they will mainly walk or canter not trot.
The only way the horse is to become balanced at the canter is to canter. It won't happen over night, but with each ride it will improve. Canter trot canter transitions are great for this. The canter will not magically be perfect without working on it so canter on!