This ignores the part where you're not sure if that same pizza will cost 0.01 BTC tomorrow or 0.04 BTC or 10 BTC tomorrow.

If I think that my 0.02 BTC could get me two pizzas tomorrow, why would I trade that amount for one pizza today?

If you think that your 0.02 BTC could get you two pizzas tomorrow, you can still buy the pizza today with BTC, and then use the $20 you saved to buy more BTC.

My point is that the decision to transact using BTC is independent from the the decision to accumulate BTC. If you want to hoard BTC because you think it will keep going up in value, then convert all your USD to BTC right now. And then if you want to buy a pizza so you don't starve to death, buy it using any currency you like, while continuing to invest all your disposable (ie non-pizza) currency into BTC.

Because you are hungry now.

Can you make it til tomorrow? Are those 0.02BTC all you have?

If you're sure you're going to buy a pizza with BTC tomorrow, why not buy the BTC today, or five days ago? The prices have been pretty consistently going up. Yeah, there's a chance that pizza will have been 5% cheaper if you bought the coin to pay for it yesterday, or if you wait til tomorrow.

So, why wouldn't you just take all of your USD savings and convert them to BTC? Wait, you mean to tell me you _only have_ that 0.02BTC? (Wait, you mean to tell me you only have that $20 in the bank?)

I have way more USD value of BTC than I ever planned on having. It started as $50 and when that performed well, I put some more in, and bought some mining equipment, and put some more in, and now it has significantly more than eclipsed my USD cash savings at this point. It is for this reason that I would love to spend BTC, in fact it's a karmic imperative for me to try to spend BTC, since I've done so well with it so far.

I can always replace it at the market rate at Coinbase. If you're spending the bitcoins and you're valuing them lower than it costs you to replace them, you're getting screwed. If things keep moving the way they are moving, you're going to have a talk with your vendor that accepts bitcoins. (It's always fun to get to ask, "Tell me why am I paying you extra?")

I make more hoarding a few meager bitcoins in the midst of this bubble than riding a desk at my dayjob. So you can see why I am in your corner.

Still, I don't think there has been a bad day to buy BTC in the last (how long has bitcoin existed so far?). I recognize that I would be a fool to keep less actual fungible cash than I need for groceries, rent, etc, and when I can use a Bitcoin in an actual transaction with a person (rather than trading it for cash at an exchange), I feel that it's probably a good idea to use some of that cash I saved to offset the impact to my BTC portfolio.

All that being said, when I watched the price take a minor tumble the other day, from 850 to 785, I got nervous and went to SatoshiDice to recoup my perceived losses in an even riskier way.

Nobody will take my bitcoins in exchange for anything I need (I can't spend them on rent, I can't pay taxes with them, I can't leave them at the grocery store or at the restaurant or the bar), apart from the exchange site (which will give me cash, that I do need)... but I have all of the cash I need for the moment and it doesn't do any fun tricks like these BTC.

So why am I buying pizza again?

I don't know if all that's a sign of a healthy market, but I don't think so. Still, you weigh the facts, you pick your horses and you take your chances.

They're deeply dependent on each other. Both kinds of user are strongly influenced by BTC exchange rates, and BTC has the same exchange rate regardless of how you intend to use it.

I encourage you to read up on the recent history of Iceland's currency.