To restate some things that nealric has already broached, the lawyer who fails ends up hurting a lot of people on his or her road to realizing that he or she cannot make a living practicing law. I've heard too many stories about people whose lawyers screwed up their cases to believe that we should just let everybody take a shot and see how they do.
Please note: this post does not express a position in terms of how we should restrict access to the profession.
Then raise the Bar so to speak. J.D.s are worthless outside of their ability to help you land a job. Bar admission should be the final arbiter, and if too many people are passing the bar and harming consumers, then raise the bar requirements.
On a completely different tract, plenty of damage done by attorneys at the top end of the spectrum as well as the bottom end.