I have heard of band directors who do that, and I think it's stupid. You are going to get more out of corps than marching band in the long run, so if your band director makes you choose, I'd say go with corps. But it's stupid of him to make you choose. Corps makes you stronger and the things you learn you will be able to bring back to the marching band. So screw what your band director says, if you want to march, then march.