I just cut and welded a new patch panel from my old hood to the under rear seat floor pan. The section included one of the drain holes. Is it really necessary that I make a new drain hole?

Well, I really don't think so if your car is going to be seal up tighter than a women with duct tape over her mouth. I think you will be fine, but if you wanted one than you are able to put one in.

The way I see it is why have a drain hole when you can use a wet shop vac.