They benefit at the expense of someone else. Every debtor who pays off his debt easier with inflation, has a creditor who is receiving less real purchasing power than he deserves. Someone is always taking the loss, with inflation it is shifted around (inequitably in my opinion).While I agree with everything you just said, some people also benefit from inflation. Like anyone who is in large debt and has no cash. Or nations in the same situation as a whole.
And the debtor, who may now be debt free or have his debt under control, is still stuck with diminished purchasing power in the long run. He's traded immediate relief for long term growth prospects.
So yes, nations can debase their currency to make the national debt easier to manage, but what they are doing is engaging in a cycle of capital consumption (eroding the value of savings) which will make it very hard for the private economy (tax base) to recover.
That's why free market people get so pissed off about inflationary monetary policy. It is just digging a bigger hole in the long run, while fixing none of the problems in the present that are used as its justification.