As a self professed control freak and someone who has flown Spirit many times with my children, I always pay to assign my seats. I can't let someone else have that much control.
I also travel with a very shy 4 year old and would hate to have him sitting by a stranger. If you aren't a control freak such as myself than I think it would be okay to have them assigned for you. I know I waste alot of money paying for 5 assigned seats.
