You know, I mentioned prerequisites briefly in my previous post. And then FSP writes a long post about how prerequisites are almost always required, blah blah blah. In the comments, she backed up a little bit and admitted that if someone already has the concepts otherwise, then maybe they don't need a specific course. Geez, you think?
This inflexibility burns me. Now, sometimes intro courses teach basic stuff, like lab safety or what the department expects from an essay. In that case, you gotta learn the basic stuff. But if you're in a department that has any claims to being interdisciplinary, you're going to get students with different backgrounds. Who cares what the name of whatever previous course you took was, as long as you have enough background that you're not floundering from day 1.
In the course I mentioned in the last post, I was missing, say, 1/5 of the expected course work. I had not taken a single one of the three (undergraduate) courses listed as prerequisites. The rest of the background I got variously from my undergrad education and from what I'd picked up at work. Did I create more work for the TAs? Maybe a little, but I wasn't asking them to teach me another course; I'd just go and ask a couple questions to clarify some background that I wasn't sure about.
I agree that you can't go and expect a lot of hand-holding if you skip a prerequisite. If you don't know a lot of the background, it's perfectly legitimate for a TA or professor to say "you need to go through this book before I'll answer any questions about review material." But if you already know most or all of the stuff in the prerequisite courses, it's a waste of time and money to go and warm a seat in a class just to get a box ticked off for the next one.