I disagree with that statement. I find that a junior dev is far less jaded by the work world and will often develop a loyalty and dedicate to the team (if it is a good environment) that a sr. Dev wouldn't. Once your are 5 to 10 years into your career how you view your work/life balance and professional relationships changes dramatically.

A junior dev will often stay with an organization much much longer than what is probably good for them, especially if they are actively mentored and engaged.