It's a fiction that money is just sitting around not doing anything.
Given money is just an abstraction that is used to allocate goods and resources that appear in the real world, someone not spending money (or destroying money) means they are letting everyone else's vote count that much more.
If a person has a bunch of money but feels they have less insight than the average person (weighted by money owned) as to how it should be allocated, then society is actually better off if they never use it.
Now the government or specific charities or someone selling things they might otherwise buy might be worse off than if they spent/invested/donated it all (at the expense of those that wouldn't have received any of the money), but them doing absolutely nothing with it is taking away from no one, and in fact raises the value of everyone else's economic "vote" of how resources will be allocated.
I don't think that encourages spending....I think that encourages offshore bank accounts and the like. Anyhow, I'd think that anyone with any type of massive windfall is pretty thoroughly invested so in some capacity it is contributing back to the economy.
I also can't imagine there are many who receive some type of windfall and are just sitting on it!
