I don't see how you've explained a problem more predominant with or unique to bitcoins. All currencies have the problem that their usage as a currency can shift. There's no reliable way to predict who will sit on their money rather than spend it. You're comparing bitcoin to perfection, not to real currencies.

versus spending them on useful trades

The whole point of money is that it has the option value to spend it now or later, so the fact that its option value isn't being exercise doesn't mean it's failing at its purpose; just the opposite, in fact. If all you care about is how often a currency is used, then the optimal currency would be spent the moment it is earned -- but that's scarcely better than barter.