I had been thinking that I'd forget about doing the whole authentication-using-MySQL thing, since it seemed like overkill for the small number of clients that my parents have. But when I talked to my mom today, she said that she'd (naturally, I guess) like to be ready to handle a whole lot more clients. And she also said that right now, she uses no fewer than three different client lists, in three different formats. And that the information they want to keep track of has outgrown those formats; for instance, she said that they've been cramming a lot of information into database fields that weren't really intended to hold that information, because there's no other place to put it. So it's sounding like they really need this new database for a lot of reasons, not just for the website. In fact, it sounds like the website is the least of their worries.
It just sort of blows me away, you know? Three different client lists? My mom is the office manager. Think how much easier her job could be!