2G and 2.5 can survive perfectly well on it own. It really doesn’t need 3G.
3G is there because people have ideas and convince other to have those ideas. It is not logical to assume that what 3G offers is what the customer actual wants, it provides what the customer has been told they will have.
3G does need GSM/GPRS for at least teh following reasons:
– fallback when 3G doesn’t work or poor, ailing or no coverage spots (thus needs 2G spectrum)
– needs 2G Mast installation for co-location
– needs 2G Customers for traffic and revenue
– needs 2G and 2.5G networks for least cost routing
Getting away from the politics and studying the technical aspects, it is quite clever the way they have got the technologies to work together if I can set out this crude outline:
using Inter Radio Access Technology (IRAT) because it requires (obviously) the symbiotic relationship to work between radio network, handset and USIM.