Okay, I'll bite...
"they've been criminally slow to take advantage of it"
Err, no. I'm already paying them for their services and as long as I pay duly pay for it I expect them to leave me the f*ck alone. I do not care about anything they THINK that I want, I KNOW what I want... I am not there to be monetized any further. DO NOT try and profile me, DO NOT try and pigeon-hole me, just take my money, provide the service, disappear into the background and NEVER, EVER bother me ever again.
However, top marks to O2 for making this opt-in and not automatically bothering me with offers of hair products I'm not going to buy and coffee I don't want to drink.
"It seems likely there will come a time when failing to opt in to such a system leads to a more expensive tariff"
Thats fine, I'll opt-in for a cheaper tariff, but don't be surprised to find I never, ever read them or even know I've got them... Apart from the first one of course. After that software for my smartphone allows me to automatically ignore and delete messages from sources of my choosing, without even getting a notification that they ever arrived in the first place.