Re: never really understood
Survival of the fittest doesn't really work if there's only one player.
Cable Vs ADSL is a really great way of looking at this.
You could say, why bother supplying cable TV when people can already use satellite? People did ask that in the 80s! As it turned out, this decision made in the 80s paved the way for fastest widely available high speed internet in the UK. No one knew it at the time. Without the cable network now owned by Virgin, BT may never have bothered rolling out BT Infinity (VDSL 2, currently offering up to 80/20).
In addition to the innovation arguments, some networks are just better managed than others (O2 comes to mind). Some networks figured out how to offer truly unlimited data (3) while others did not (O2's network colapsed a few times from excessive iPhone data usage). IIRC Orange was the first to offer HD Voice services over it's network. With shared infrastructure not only would innovation be stifled, the lowest common demoninator indulged, but everyone would be at the whim of one organisation to get it right.
What about the benefits of not duplicating infrastructure. People oversimplify this, thinking that one tower can handle an infinite amount of people and it can't. Companies that are well run will have spare capacity, but they wont spend loads on having a lot more than they need. I very much doubt that one company's infrastructure could support everyone in the UK. Particulalry if that company was one of the smaller ones. So this idea that we can save towers starts to go out the window.
Duplication of existing services in different ways leads to the best way of serving the public at the lowest cost.
Businesses will start sharing infrastructure when it is in the public good (selling good products and services means more profit to them) and they have and they are.