There's also the point that an increasing number of outfits that I do business with online demand a mobile phone number, usually for no good reason.

Oooh, that's annoying.

I've had online businesses require a phone number (not a mobile number) because the courier required one -- but there was a reason, that is so that the courier can ring me if they have a problem, which actually happened once, because the label on the parcel was wrong, it had the wrong number for the street number, and they were able to ring me up to sort out the problem.

But I intend to guard my mobile number even more vigorously than I guard my email address; no way am I going to give it out willy-nilly so that I can be spammed with ads (which is apparently a common problem?).