Serve a useful purpose, but are not accepted by all browsers. If you're set on using them, I'd recommend at the very least you run a browser check and then render content to those browsers in some other means.
Pages with blank content areas don't get readers too excited.
At least the most recent Netscape versions accept Iframes.
Cheers,
Mark
BTW, it's the 1st and your back up. Poor practice by a host shutting you down for going over. Mine just bills me the additional bandwidth cost.