Ed - a couple of things. If its not about money, what is it about. I like appraisal, but will not do it for free. So, for me it is about the money.
Also, how exactly is it unethical to do a drive-by? I do very few, and don't like doing them because it is hard to get data. However, If the customer wants one, and USPAP says it is ok, then what is the problem. USPAP doesn't even require leaving your office to do an appraisal. You just have to say you never saw the property. I personally wouldn't do that, but it isn't necessarily unethical.
If you aren't comfortable doing these, don't. I am assuming from your post that your mentor really doesn't approve of them. I would also assume from that stance that he/she is likely a very ethical appraiser who has very strict standards. Remember, however, that just because your mentor doesn't approve of something, doesn't necessarily mean it is unethical. There are different views on many subjects, and as long as folks are performing their jobs within the bounds of USPAP, they are not being unethical. Basically, all the clients want in this case is to know if the house is worth x, or more. They couldn't care less what the actual market value is.