yeah there is some logic to that. Certainly easier to implement ;)
I guess thinking of business rules - nulls are evil things. Even in a database, they are evil. Many an important report is incorrect cause it is built on a database that allows null values, and people don't understand the ramifications of the SQL statements used.
Nulls in your fact model: just say no !
of course... back in the real world...
Some other thoughts:
Foo(bar < 3) makes no sense if bar is null, so it should be false always in my opinion.
However, Foo(bar != 3) is not so clear for null behaviour.