"but it doesn't seem necessary for one simple banner ad. "
Yes and no. If you're using Classic Text that is Static and not Input or Dynamic then you don't need to embed fonts. As of CS5 it is more commonly required to embed fonts if you're using the latter two though there might be cases where you are lucky and don't have to.
If you're using TLF text it's a whole other ball of wax. In many cases even embedding fonts when using the TLF Text Layout Framework engine will still not work to produce your fonts. However it is mostly required when using TLF to embed fonts. You can embed via the interface or optionally via ActionScript though I don't know exactly how to embed fonts via ActionScript just yet. I've tried but didn't get satisfactory results and had to resort to embedding via the Flash interface.
Hope that sheds some light even though you might get better information from someone else who might post.