I am sure a neatly pulled, hunter style mane looks bizare to anyone who's experience is entirely with western horses.
We all carry prejudices based on our early training, but it's important to recognize them as not entirely rational and have a sense of humor about it.
The very best reason I can think of for showing a hunter or jumper with a long mane is that the horse also competes in breed shows, where the convention is a long, natural mane. Another reason may be that the horse is one of those that hates having it's mane pulled and needs to be sedated for it, and the owner feels that it isn't worth it for an occassional schooling show.
All that said, my irrational prejudice is that a long mane on a hunter, jumper or event horse looks odd, and I would hope that someone wanting to show seriously would respect the conventions of the sport.
However, I think "repulsive eyesore" is overstating it quite a bit.