This is interesting theory, but what about situations such as work orders which are not complete but have a DateCompleted field.  That item is simply not know, and using an arbitrary date such as 1/1/1900 is simply not acceptable.  And pulling up incomplete work orders with an elegant WHERE DateCompleted IS NULL is easy and it makes sense.

I appreciate that there are pitfalls with using NULLs which require care, and that sometimes large numbers of NULLS can reflect a poor design, but I remain unconvinced that eliminating NULLS is the hallmark of good design and programming.