I generally dislike nullable columns, lots of nulls usually mean a poor design, however, sometimes you don't always have all the data - and lets not forget about apps that create new rows by inserting a blank row with the PK ( usually a guid )  and then update the newly created row.

So if you don't have all the data you need a value , maybe null or maybe a default, such as 'not set' or 'unknown' etc.etc.

I'm slightly confused about the overwriting of a default, a default is usually used to give a value where one is not provided at the time, for instance a blood group ( well I figure not all people know their blood group ) in a doctor's system.  The defaults such as dates, userid or such - why would you overwrite them - your documentation and data dictionary should cover this and your test and release process catch any issues. ?