Can you explain why it is worthless? I'm a little curious.
I'll take a stab at answering this.
Most applicants shadow a doctor for at most 20-30 hours before moving on to a different specialty (past that, it starts to get really boring and isn't that beneficial for your app anyway). During that time, the doctor will get to see 2 things
1. How well the applicant can stand in a corner and be silent
2. How many basic questions the applicant can cram into any dead time
So why would a letter from a doctor be particularly useful? The doctor doesn't see the applicants academic performance nor does the doctor really have enough information to compare the applicant with other applicants. The doctor doesn't really get to see the applicant interacting with the patient besides "Hello" and "Goodbye".
Compare the situation with a science professor who may see hundreds of premeds in his or her class every year. The professor can see how the student relates with his or her peers and can see the academic performance and potential of the student. Nonscience professors may not see as many premeds, or at least don't know they're premeds, but they still get plenty of students who want graduate degrees and they can give the adcom a good view of the student in a different environment from a science classroom.
Other good letters could come from PI's over the applicants research, supervisors over a group of volunteers, and sometimes from employers (there are others of course). Notice that each of these categories involve a substantial interaction with the student. The interaction is much less superficial than the physician you shadowed for a few days.
If you get involved with a clinical research project and a doctor is leading it, by all means get a great letter from the doctor. I would also say if you find a doctor who "takes you under his wing" and mentors you for a while, then it might be appropriate to get a physicians LOR. However, this is not the case with the majority of applicants.