I'm not sure if this is related or not, but I do know that having the ability to have a "null" (empty) field is a basic database function, and is even more important here. The bottom line: it means that neither the join form or search functions work properly on D7. It looks like Alex has come up with a workaround for the problems this creates on the join form, but if you use it your search functions won't work. I don't think this is an obscure "feature" or "enhancement" see more - I think it is a mission critical function, and as the communities getting build with D7 get more developed, I think that more and more members here will agree.