If you know your sizing and are comfortable with the basics of bicycle mechanics, it's a good idea. Or if you want to use the beater as a springboard to learn basic bike mechanics, it's an excellent idea. So as a learning process, I don't think it's a bad idea. Newer bikes do have some different systems than older bikes-- in particular sealed bearing hubs and bottom brackets and indexed shifting, but many things, like cable and brake adjustment, chain care, wheel truing, etc., remain unchanged.

In general, look for lugged and butted older road bikes; they're harder to find than an all out department store junker but not impossible. Check your area for charity auctions; for example, the Salvation army here auctions off piles of bikes a couple times a week.

Be prepared to buy some bike-specific tools, as well.