I agree. Most, if not all, schools require that the professor actually be one of your instructors. The letter would need to mention this as transcripts will not generally show this to the school. I've seen letters where it opens with something like "I've had this student in bio classes every semester since time began and s/he consistently is in the top .01% of all students."
It can be a real problem to get a decent letter because some schools have classes in excess of 200 students and it may be difficult to show up at the office hour, or to speak with the prof after class. Most dental schools won't take a letter from a TA. If you have a pre-med committee then it might help to get a letter from the committee as some schools will take either a committee letter or three different science prof letters.
As a non-trad student the science LORs were the hardest part for me. I graduated and didn't keep in touch with most of my profs. Then, out of nowhere I needed LORs. Luckily they were understanding.
