To me, UBI is a solution looking for a problem. As you say the common problem definition is it is needed is because “there won’t be any jobs left anyway”.
I may have bought into that a year ago but I have just finished a year of research on automation trends over the last century for my book, Silicon Collar. I was staggered at how many millions and millions of jobs — postal, grocery, secretarial and many others which were predicted to die off decades ago continue to survive in large numbers — my ZDNet post at http://zd.net/2cwdFhX discusses several. So in my book, I analyze that our societies have what I call “circuit breakers to over automation”.
Not only should we not panic about imminent massive job losses from automation, I also found in my research of BLS and IRS data, a vibrant labor economy. The BLS has reported for 3+ years 4+ million unfilled jobs in each of its monthly reports. The IRS reports if you exclude the top 1% the rest of us plebs still report $ 10 trillion in income a year. A growing % of that is coming from new gen jobs — franchises, platforms ( iStore, Amazon fulfillment, eBay work at home, Uber etc) and a whole new categories of services — alternative health like acupuncturists, herbalists, pet care etc — none of which our parents enjoyed.
So, to going back to the point — “there won’t be any jobs left anyway” — if that the problem UBI is trying to solve, it may be decades if not centuries ahead of the problem.