Re: I could never understand..
We're almost half way to that anyway, between Vodafone-O2's "Cornerstone" and EE-Three's MBNL.
I'd be happy to see Cornerstone and MBNL pooling resources to cover more remote areas - though of course, with one part of MBNL buying part of Cornerstone, things could be messy for a while now.
I would be wary of too much consolidation, though: having watched the coverage of different networks, it's largely the case that a gap in one network matches a gap in the others anyway. Most of the exceptions seem to be where a network is having technical problems (dead backhaul, failed cell kit, congestion): if that equipment were shared across networks, that would mean (a) more customers affected and (b) no ability to bypass the problem by switching networks, as dual-SIM handsets and roaming SIMs can do right now.