I've never liked it, myself. A lot of people (most, probably) don't know what it means, because it's not particularly evocative of its meaning. Those three words are cut off, a fragment of the longer and more complete explanation. 'Taking as a given what remains to be proven' is a longer statement, but it has the virtue of being crystal clear. I don't think I would ever use 'begging the question' if I could make myself that clear instead.
In reviewing a film about a 1960s movie director and the actress he married, Tobias Grey in The Spectator writes of 'their May-to-December romance (Godard was 36 and Wiazemsky 19 when they married)...'. The fate of the romance is not the issue here: what grabbed my attention was the umpteen millionth iteration of that cliche, 'May-December romance' (here given an unneeded preposition).
This trope annoys me because it's sloppy.
If the man wasn't even twice the woman's age, how is he December to her May? January to May is five months inclusive. Double that and you get to October: at the most, this was a May-October romance.
If a partner about twice the age of the other is 'December', what on earth does that make real geriatrics such as Mick Jagger (in his 70s, with a 30-year-old girlfriend) or the late Saul Bellow, who was in his 80s when he rather gruesomely fathered a child with his just-middle-aged wife?
Is Rowan Atkinson, a lithe and vital man as well as a money-bags celebrity, 'December' to his girlfriend's 'May'? Hardly: he's all of 62. If anyone dared to call me a 'December' type at 62, I'd remind them that the only way it fits is that I'm Sagittarius, at which point I'd offer to stick the arrow in.
Perhaps most writers can't do maths. But they ought to try, and make their figures of speech add up.