Most LORs are from undergrad professors, and usually at least one undergrad professor is required. While an LOR can be from anyone at all, it is hardly the norm to have a law professor write one. What would a LOR from a law professor even say about you? I certainly think it would be a nice one to include, and one that most people don't have, but he is essentially recommending you based merely on your personality or or general competency. While his position is noteworthy, it is also balanced with less relevancy. It also has the appearance of being a recommendation from a "friend" and perhaps a little less than completely objective. From an admissions perspective, who could be better to assess your academic capabilities than the professors who graded you and saw you in a classroom on a daily basis?
I would definitely include any LOR from a law professor, attorney, judge, etc. Of course the more important or respected they are the better. This is a nice touch that will help to set you apart from many of the other applicants. But the meat of your recommendations as far as adcomms are concerned are those that can testify first-hand on your study habits, attitude towards classwork, writing and discussion skills, and potential for academic heavy-lifting.
Put it this way: If you were a law firm looking to offer a job to a graduating law student, would you value more what his law professors have to say, or a lawyer that he knows at another firm?