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.
Your right about day to day (even week to week) price fluctation being annoying for buyer and/or seller.
To me this is simply a symtom of this new currency being adopted world wide. It is going to take some
time for these day to day bumps to smooth out, but I'm confident it will eventually happen for short term fluctations.
My speculation is that it will take serveral more years.