I've started taking spinning classes at my university's gym. I haven't been in a spin class for maybe four years, so maybe there have been some new discoveries about the best way to do it, but I'm confused about the reason for a difference in form.

That is, the instructor wants us sitting straight up when seated, not holding onto the handlebar unless we need to for balance. (Though in a seated sprint he did say we could grab the bar.) I can do this and it doesn't hurt. But it doesn't seem right either, though maybe that's because of my road riding background. I know there are some spinning instructors here, so what you say? Is this sitting-straight-up form pretty standard practice in spinning, or does it sound unusual to you? What are the benefits?