I'm a bit baffled by how you think turning kilograms into decigrams is 'a right royal pain', multiplying by 10000 seems pretty simple to me.
That's performing arithmetic (and you're of course correct: in base 10, multiplying by 10,000 is really easy — just slide the decimal point right), not manipulating a physical quantity. Take a gram of sand, and try to cut it into 10 decigram-weight quantities using a balance without a pre-existing decigram weight. You're going to have a hell of a time: there's no good way to divide a physical quantity of stuff (whether a volume, a length or a weight) into tenths. It's much easier to divide things into halves and (less easy) thirds.
Thanks for commenting, though. I wonder if others misread similarly (and I've added one word to clarify). I can't imagine anyone seriously believes that it's easier to divide quantities of physical substances into fives or tens than into halves.
(as an aside, the HN rate limiter has been insane this week: I posted one article and three comments, and I've not been permitted to post for over an hour now)
Granted, if we look at the tiny details, the exact measurements of each size are not what one would call intuitive, round numbers, but it's much easier and reliable to use than the US, non-decimal based standard.
I think it's mostly an economic problem, for the majority of people it's too much trouble for very little real gain. For engineering, on the other hand, it's almost mandatory.