Was thinking about this while cooking eggs today. Yes, we "should," but no, we won't. "Bitcoins" have no tangible value, so we have to compare it to something we can use. I used to use "McDoubles," but that's really more related to valuing USD, not BTC (at least not independently). If McDonald's accepted BTC and didn't just rely on exchange prices, then they could effectively set the value of BTC for me, but for now, I rely on speculators, like with all currencies.
For virtually everything you can buy with BTC, you can buy with USD, so it makes sense to check which one "costs less." If they don't match and it isn't a big deal, you may's well buy using the "overpriced" currency.
Or, in a way I'm more able to relate to.... If McDonald's sold McDoubles for $1 or .5BTC, and I could sell .5BTC for $50 on Gox, I'd obviously want to pay for my enabled suicide using dollars, and I really don't give a shit about the stability of Bitcoin to the point of effectively paying 50x more for a cheeseburger!
Hashpower is effectively determined by price, so determining price by hashpower (more competition = less BTC per hash) would be ridiculous. All we have are each others' emotions and gambles, and that's true with all currencies (except the cost barrier to changing most fiat currencies like USD is much, much higher -- tut makes a good point).