Redefining NULL on a whim? You can't redefine something that doesn't have a good definition to start with. N/A, unknown, not yet entered, incomplete, etc. can all be valid interpretations of a NULL depending on the context. That's the problem. In order to use it, you have to have a potentially different contextual definition in every table, in some cases multiple definitions per table if you have multiple NULLable columns. Even those can be wrong. Imagine a system where a client's address was NULLable. It might mean that the address had not yet been collected, or it could mean the client was homeless and had no fixed address. The meaning of the NULL address would be ambiguous.

I avoid using NULLable columns where practical, but I'm not religious about it.