I wouldn't be without mine; but I don't think it's unreasonable not to have one if you don't want one. Unless, that is, you ask others to use theirs to make 'a quick call', or to take a photo and send it to your daughter, as your phone doesn't have a camera - oh, and daughter doesn't have WhatsApp, so sending it costs 50p for a text message, or take a message (or 3) from your son when his train is late, so you don't waste time at the station, or . . .
I'm sure that you wouldn't do any of these things; but I have a friend who has done all of them, and more. She doesn't use Facebook or Twitter either, which is fair enough; but she complains bitterly when she can't easily access things that are on social media, such as special offers, or tickets for things.
I have pointed out that it's up to her whether she uses them, but as (a) she does have a computer, (b) both Facebook and Twitter are free to install, (c) she can use them as and when she likes (so neither is intrusive), it is like someone refusing to have a landline and expecting other people to knock on her door when they want to contact her.
So overall, whether UABU depends on whether you really don't want to use one, or whether you don't want to own one.