The generally-agreed rationale of UBI is that growing use of automation might eventually make it hard for people who lack relevant skills to support themselves even by working

That's an often-quoted reason, but it's far from "generally-agreed". For one thing, there is the obvious retort to it: we'll consider UBI when the robots actually make people unable to "support themselves even by working" and not before that.

A lot of people (e.g. Charles Murray) support UBI as a less-painful alternative to the metastasizing bureaucracy of welfare, etc., and with less mis- and disincentives, too. It is basically seen as a welfare equivalent to the "taxes on a postcard" (or maybe flat tax) movement.