Good point, although GeoIP only works for home broadband really (and even then, only up to the nearest block or so); on mobile networks you only ever get to see the GGSN's IP no matter where you physically are (the idea that each tower has its own IP seems like a myth to me, never seen that actually happening for real), so you're missing out tons of info.
On the other hand device-based location (like the one that gets attached to photos) is often accurate enough to tell two flats apart so you can even tell very close users apart.