^ If you are training collection and higher level movements, yes you will certainly create bulk that may be detrimental to performance in endurance.
However lower level dressage, as was specified - things like rythym, relaxation, straightness, yeilding, etc. - Are all 100% beneficial to an endurance horse and will only help, not hinder, performance.
An endurance horse that is lean and light and only knows how to travel straight down a trail all strung out is going to get sore and stiff much more often than a lean and light endurance horse who has learnt how to be supple, rounded through the back, more efficiently carry the rider and move it's body.
Learning to carry more weight on the hind and strengthening the hind are fantastic things for an endurance horse.