It does seem an odd metric to measure how big a network is.
I would guess City analysts are looking at this though, concerned that in some mythical future of hyper-fast universal 4G, the winners will be those with the bandwidth to sling 4K movies to peasants casually watching whilst on the bus. Factor in the madcap ideas sometimes touted for the Internet of Tat, smart homes, smart cities, networked self driving cars and all the other fanciful crap, and you can see WHY they are thinking that, even if you don't agree with them. Note as well that everybody involved benefits financially from a "successful" high priced IPO (except customers, and the new owners). So O2 management's options rise in value, their bonuses for the IPO get bigger, Telefonica stroll off with a bigger bag of cash, the investment banks, lawyers, management consultants and others all take a much bigger slice of cake.
And in the situation of an IPO, it'll be things like that future potential that are bigged up in the prospectus to investors, as this huge unmissable investment opportunity. Come on in! Fill your boots! The water's lovely! Of course, when the new owners throw open the barn doors, they'll be in for a shock at what's inside, and they will also have to sort out the commercial mess made by caning the business for results. Which will be even harder than it is now, because all the multi-billion pound deal costs and acquisition premium will be strung round the new O2's neck like a bloody great albatross.
If you're currently a customer of O2, be prepared to find another network after the IPO, when they need to increase their prices. As an example of the impact of corporate acquisitions on customer prices, take the Liberty Global takeover of Virginmedia: I've seen my bills go up by 10% each of the past three years - I'll bet VM employees haven't been seeing double digit salary increases.