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http://alexsegura.tumblr.com/post/8691920116/my-hat-is-two-s...Think of it this way. If I told you I was keeping my savings in bitcoin, and I've saved hundreds of bitcoins for the last two years, and never converted any of them to cash, and never spent any of them in the last two years, you would probably say "Wow. You've missed at least two great opportunities to cash in on terrible market crashes. You could have had thousands without investing any more real money, you'd be filthy rich and have placed nothing at risk."
Certainly I could have done better if I had made _some_ trades. History tells me I personally will pick the wrong times to trade, and the above strategy is probably best for me, given my strong propensity to buy high and sell low (otherwise I would have those hundreds of bitcoins still.)
But if you've never made a transaction, you have never realized any of the value, and an argument can be made that your entire portfolio is at risk.
Just like buying 50-year bonds, there is a risk you won't live that long. Nobody's going to contact your next-of-kin to alert them to your bitcoin holdings if you die having kept them a secret. That's a real danger to the network, and with a cap on the total number of bitcoins, it causes real harm to everyone when those coins are lost, unaccounted.
At least with someone moving them around from time to time, you will be able to know that the keys have not been erased and "those coins are still alive," you won't spuriously inflate the value of your own and your neighbors' coins on public exchanges, opening yourself to the real dangerous Satoshi attack of "rising from the dead and seizing all of the at-risk cash on everyone's trading accounts." If Satoshi ever comes back, we will likely have made him fabulously rich and all because we didn't think of it first, and people like to jump on bandwagons.
No contract! No fair. Ergo, demurrage is good.