Effective speaking before a strange audience is as much about power as it is about communication. If you look weak, sloppy, or disorganized, you're going to have a bad time. Sitting down certainly doesn't help you look strong, neat or organized, and in the worst case, it probably hurts you.

Ultimately, this fellow's argument sounds a lot like those people who worry about being "too formal", and therefore under-dress for every occasion. If you're in doubt, dress up and stand up. Neither can hurt you.