I would strongly disagree and Joel offers no evidence in defense of that statement. I wouldn't expect junior employees to have a strong understanding of task complexity, their own ramp-up period, available resources, unforeseen impediments, the experience necessary to estimate accurately, etc..
Estimates are best done by the team as a whole as it is the only way to rely on everyone's collective judgment ("I don't think we need this requirement," "we are going to run into this problem," "this is similar to task X we completed 2 years ago"). If done in this manner, the process also serves as training opportunities for the more junior employees.