Great article on NULL usage.

I find NULL's in a database an invaluable tool, for many reasons... And easily avoided with not null constraints and a default value, where inappropriate.

In addition to making things more complex, NULL's also make a number of operations simpler, for example, the avg() function by default nicely excludes null values, causing an average of only known values to be returned.

Handling NULL's according to the application of the data in question is generally a simple matter with functions such as coallesce and isNull; and without the ability to store a NULL, that is valuable data lost. With a NULL stored you have the information to handle the data precisely how you want to.