I guess things must be different here in the UK, but this seems like a conversation from about twenty years ago. I would be pretty much amazed if I met a working musician here who didn't have a cell phone (or almost anyone else, come to that). It would seem totally weird.
I'm genuinely surprised that there are guys on here who say they choose not to own one. I'm not saying that you're wrong to do so - it's entirely your choice, of course. I'm just saying that I find it really, really surprising, mainly because I would never expect to find anyone in that category over here.
It's often very helpful to be able to contact band members before arrival at a gig, between set up time and the first set, during an interval... whenever. For all sorts of different reasons. But I can't imagine saying to a prospective band member "It's a condition of joining the band that you have a cell phone", as that situation just wouldn't arise.
On the other hand, I know a few people who don't have land lines. I do, but I never use it and never give out the number to anyone to call me on it. Here, owning a cell phone would definitely be a bigger priority for a musician than a land line, every single time.