I would suspect that most Bitcoin advocates would disagree with you. The simplest counter-argument is this -- wouldn't that mean inflation discourages people from selling things? And you can't spend if nobody wants to sell to you, can you?
Another way to make the same counter-argument is this: Actually, no, deflation encourages people to spend the bitcoins they have. A deflationary currency should be very desirable to hold. So, if I have a deflationary currency, sellers should go out of the way to encourage me to give that currency to them (with low prices, easy transaction, stronger guarantees, whatever it takes). That will make me more likely to buy things.
Of course, all of these arguments are nonsense. The characteristics of the money affect the buyer and seller equally. So while they may change the price, they won't change the likelihood the transaction will occur.
But it won't even matter. Long-term predictable monetary deflation is impossible (for a low friction, demurrage-free currency) because it entails a direct contraction. If you assume long-term, predictable monetary deflation, you get the following paradox:
One unit of currency today must be worth less than one unit of currency tomorrow. Otherwise, you don't have long-term, predictable deflation.
One unit of currency today must be worth more than one unit of currency tomorrow. This is because one of the things you can do with a unit of currency today is hold it until tomorrow. But you can also spend it today if you want. So it has to be worth at least as much.
These two arguments directly contradict each other.
Yes, you have been lied to by economists who want to convince you that the economy will explode if the government doesn't enrich itself and favored bankers at the expense of everyone else. That's not to say deflation can't cause real problems, just not this kind of deflation and these kinds of problems.