There's no good explaination. Trying to force people into spending their coins is always a bad idea. No one wants a deprecating asset.
There's value in an inflating currency if the inflation pays for the network security, allowing cheap transactions on a secure network. A sidechain like freicoin which people occasionally moved small amounts of coins to would be useful.
Variable monetary inflation--increasing the money supply--is nontrivial, and requires a trusted issuer (central bank). Many here don't like that.
Algorithmically controlled monetary inflation (e.g. Bitcoin inflation, ~10%/yr now, tapering as more blocks are found) is, by definition, as inflexible as demurrage.
You've described one way in which demurrage is no worse than inflation. My question is why people think demurrage is BETTER than inflation.
Obviously they're equally inflexible. It seems the following is true:
(1) The actual monetary effects demurrage vs. a constant fixed inflation rate are identical.
(2) Most people understand inflation, but are unfamiliar with demurrage.
(3) It's harder to know your exact purchasing power with demurrage, because to find that out you need to do a calculation using the age of all your coins. (Obviously wallet software would do this automatically).
The only potential benefit I can think of is that maybe demurrage is somehow easier to implement technically on a side-chain. Maybe not.
Can anyone point to any advantage of demurrage over inflation?