Some folks are not fans of having NULL values in your data. I am not much of a fan myself, however sometimes (all the time) they are a reality in which we have to deal. When working with NULL there are lots of strange behaviors you have to be sure to manage, for example WHERE x = NULL gets you nothing, whereas WHERE x IS NULL gets you the data you want (which will still depending on your ANSI NULL settings). Not much fun to be had with NULL.
Something else to be careful of is managing queries where you might look at a value in one column, and if it’s a NULL, return the value from another. For example: