Some characters' ultimate weapons - Cid's and Nanaki's, if I remember right - get stronger as the character's MP increases. So if your team's at relatively low levels, but you want these weapons to achieve their full potential, then HP->MP materia would be a good way to do it.

I've never tried this, so I've no idea if it actually affects the Venus Gospel and Limited Moon, but I don't see why it shouldn't work.