At the very least, the mentor system serves in two ways:

1. The mentor roulette is another means of helping fill in the spots for queues.
2. New adventurers and returners get bonus experience points when partying with mentors up to level 20.

The part where the mentor system might be lacking is also twofold:

1. Mentors do not give advice or give wrong advice to new or returning players either in novice network (assuming they even join) or out in the field. (I added the target audience because I don't think a mentor is necessarily supposed to be an expert at everything, but at least know enough of some of the basics).
2. Mentors refuse to stay and help do a particular duty. (Going back to the part about not necessarily being an expert, if they've cleared the duty, then they should still be able to help by staying and doing it even if they don't know the full mechanics of the fight.)

At this point, it depends on each individual mentor whether the benefits outweighs the negatives. There are certainly mentors who live up to what should be expected of them, so I wouldn't say the system is useless.