Stresses the hose and it's not easily visible from the front (I've been putting it on right hip D ring, not chest D ring). Those are the only reasons. Those of you getting all worked up about it and saying to switch shops/instructors are overreacting. GUE goes all standard, why is it a bad thing if an "mainstream" instructor wants the octo up front and visible on a more "mainstream" method?
Sheesh, next thing you're going to tell me I'm going to die because I'm diving splits with a drysuit, and I'm not going to be able to propel myself through the water well enough.

Balderdash!

My knees don't hurt for the first time while diving, and that's the most important thing.