It's not that a wax ring with the horn can't work, it's just that it can create problems. Lots of them are installed and work fine. But, they cost a little more and create problems more than just the simple wax ring, so if yours is working, not clogging or leaking, leave it alone. the problem comes if the flange is high, or the opening is narrow - the horn can then distort and almost literally close the outlet off. If the flange is too high, you might squeeze all of the wax out and just have the plastic, which can leave a path to leak.