Music Quality. First and most obviously, you're buying better quality music. The new 256 kbps AAC tracks offer twice the bitrate of the current DRM'ed selections. More bits mean that the music will be more faithful to the original audio quality. Can your ears really tell the difference? It depends on the kind of listener you are. My sister buys high-end speakers and goes on about the audio experience. Me, I still listen to audio tapes I've ripped to iTunes.
Interoperability. No DRM means that your music will play back on many more platforms, like the Zune. Of course if your media player doesn't support AAC, you're kind of out of luck unless you want to convert your music or buy a better player (which the lack of DRM makes possible). Interoperability also means you can better take advantage of fair use in other media like videos.
Best of both worlds. For your thirty cent upgrade, you will presumably own both the original track you downloaded as well as the better quality larger track you upgraded to. If you own a small shuffle as well as a larger nano, iPhone or video iPod, you might be able to create separate syncs to take advantage of the space-versus-quality versions of your tracks. This means a lot of extra work and it means you will need to buy your music twice. At least until Apple discontinues its 99-cent DRM pricing model, which is a door that these new $1.29 tracks opens.
Convenience. It's not exactly a secret that you've long been able to burn your iTunes purchases to CD and then rip them back without DRM. But for thirty cents, you can now skip the burn/rip step and save yourself a bit of time. If DRM-free music has an intrinsic value to you, perhaps those thirty cents isn't too high a price to pay to skip the work of doing it all by hand.