null or not null

Ok guys, I need your help on this. I recently had a "fight" with my supervisors ( who never worked as DBA ) for "null" and "not null". According to them, they want to change all the "null" columns in database to "not null" columns and put a "default" on them. All they want me to prove is, why can't they do it ?? Space is not a problem here. But will the performance will get affected by this ? I really need to have some points to fight them and surprisingly I am running out of them. Please help !!! Thanx in advance.