That is the topic considered here. The ABA's big argument in favor of making law school mandatory is one of public policy. If the ABA requires that a lawyer attend three years of school from an accredited college, then it cuts down on the number of incompetent individuals who would otherwise be able to sit the bar. In the long run, it argues, the standards protect the industry and the public.
Others think that the strict requirement of three years for law school might be only necessary in the eyes of the ABA, the full time professors, and the law libraries, who rely on full time, three year law students' tuition to pay their salaries.
Now, I don't know that law school is exactly necessary to be a good lawyer... Among others, Abraham Lincoln and Daniel Webster never went to law school. However, I think that a mandatory law school requirement does cut down significantly on the number of people who arbitrarily decide to try to be lawyers without regard to their capability or what is required of a lawyer. I submit that for every great lawyer that came from not attending law school there are no less than three completely incompetent ones. Having a law school standard cuts down on that significantly due to the deterrent factor.
ABA accreditation is a different thing, and something that I can't comment intelligently on. I've not worked with any lawyers from Concord School of Law to know how qualified they actually are. I don't know that I'd want to have a person who got their law degree online representing me, but that speaks for the means, not the individual. If any of you have experience with lawyers who received their degree online (to the best of my knowledge, only California allows them to sit for their bar at this time), let me know... I'm interested.