I have dealt with DBA's who insisted on all columns being non-NULL and I have engaged in "disucssions" about such (both in person and in cyberspace) and I have yet to find a really good argument for changing my position: NULLs should be used when appropriate and avoided when not apporpriate.

If there is a referential integrity rule that, in effect, says that every row in TableA has to have a FK/PK link to TableB, then it is inappropriate for that column in TableA to be NULL. However, if there is a column, for instance, in a Hospital Information System's PatientDetails table, that is supposed to contain a Date of Birth and it is possible that a) that information may notbe readily available (e.g. John Doe is unconscious when admitted) and b) it is not a show stopper if it either is incorrect or unavailable, I would rather have that column in that row left NULL than to assign an arbitrary DOB. If the column is not essential to the overall integrity of that row, when I am faced with the choice of using an arbitrary default value or leaving the column NULL, I choose the NULL.