I agree with the OP that eventually the stability of bitcoin won't matter. Most people won't hold significant wealth in BTC (unless speculating) and services that guarantee funds @ certain external currency will spring up.
i.e.
1) merchant wants to sell a widget for $16.
2) Realtime exchange rate API puts exchange @ $8:1BTC and thus price on website is 2BTC.
3) Customer buys widget for 2BTC and wbsite API immediately locks in $16 USD for 2 BTC with "guarantor service".
4) Merchant is guaranteed funds in dollars regardless of how market moves.
Added bonus - speculators can now make money two ways (one currency moves & collecting fees on locking in exchange rates)
This doesn't address the issue that if the market moves up, it'll turn out my 2 bitcoins were worth more then 16 usd next week, and I'll feel I paid more then my widget was worth. If for example, the BTC value for me the day my widget arrived was $10 : 1btc, I'll feel that I could have had 4 more dollars, if I waited a week. This happens gradually with any currency, but when I'm looking at BTC price histories, I'm seeing pricechanges by over a dollar a day, and not infrequently.