The ADA uses the language "reasonable accommodation" - which can be almost anything that will assist the worker or student in a way that doesn't pose a signifcant cost or risk for the employer or school (not exactly a thorough legal description, but you get the idea). So yes, extra time is a reasonable accommodation. If the dentist is not self employed, but is employed by a corporation, they could provide extra time with an assistant to help the dentist go over files and color code them (if he is self employed, hopefully he'd have the common sense to put this in the job description of an assistant). Basically, if someone can perform a job in a fully competent way by making some minor physical or staffing adjustments, you cannot disqualify them from a job or school. If the disability is serious enough that nothing can compensate, then disqualification (yes, based on a disability) is legal. But that is almost like saying that I, five feet tall and about as athletically coordinated as a baby giraffe, ought to be able to walk into a job as an NBA coach if I wanted to.
I was diagnosed with ADD in my 3rd year of a 3-year program in grad school at a competitive (Ivy) school, at the age of 30. I had been doing OK all along, but at one point that fall, the pressure became too much, so I saw a psychologist at the school who specialized in such things - it was reassuring that the university is was at had enough others with LD/ADD problems as to hire a guy devoted to those disorders! I disclosed the problem to the student affairs department, and I was given the opportunity to take exams in an empty room, and take as much time as I wanted. The empty room was definitely a benefit, and I never needed the extra time when I could take the exam alone. However, I also disclosed to my advisor, and things got kind weird after that. He said "Oh, that's going to have a bad effect on your career," and he seemed to drop me from his roster of "good students" - the ones who got good TA opportunities, spring break research trips to Paris & Florence, etc. So my benefit was mixed.
To the OP, I would say that it is worth it to take practice tests - even postpone your application a year if you have to in order to get a handle on your capabilities. Somewhere along the way, I was advised not to ask for the extra time, because the MCAT score is flagged, and adcoms will know right away that there is something afoot. You will get in to school based on your accomplishments, and if you can swing the MCAT, don't bother with the accommodations - however if it is going to mean the difference between a 20 and a 30, I think that a 30 with accommodations will look better to anyone. If it is the difference between scoring a 30 and a 35, I don't know if the benefit is so clear cut. Schools are not supposed to be able to discriminate based on disability, but they are looking for easy reasons not to admit someone, and your application will become less competitive... however unfair this is. And, if you tried to pin them down on it, they could always point to something else on your transcript - I think we have all seen examples of the apparently arbitrary nature of the application and admissions process to realize that they probably could find something in anyone's transcript that would make the applicant less than desirable for some dumb reason.
As for the theory that given extra time, anyone could do better on the MCAT, all that would do for me is give me extra time to change my answers to the wrong ones. I work much better under a time constraint - helps me to focus.