I didn't say that I didn't care, just that the letter would be acceptable. Yes, we like a glowing letter from a full professor (or a distinguished professor with a named chair) but we hate to discriminate against students who went to big state colleges with >500 in the lecture hall. If the best that the full professor in that case can write (if there was even one remotely associated with a low level undergrad class) is something along the lines of "I don't know Joe Schmo personally but he was enrolled in my Organic Chemistry course last year. The class has over 500 students. He received a 92% (top 5% of the class) on the midterm exam and had the best grade on the final, a 98%. My TA tells me that he seldom attended office hours but I assume it is because he didn't need to. Mr. Schmo is a fine student and will do well in any academic setting he pursues". Really, am I going to get anything out of such a letter? I'd much rather have a letter from an instructor in a class of 10-30 students who actually got to know the student's personal attributes.
But, you make do with what you can get given your circumstances.