It never will be a currency either - it's deflationary nature is a huge, impossible disadvantage. Why would anyone use a bitcoin to buy bread and milk when (by design) that bitcoin will be worth more tomorrow, and more the day after that, and so on forever? Unless there's some good you can only buy with bitcoin there's no reason to ever spend them instead of cash money dollars.
You can get the same effect by investing dollars though, each dollar you invest ends up being worth more with interest. You could ask the same question of why would anyone spend dollars now, when if you invested that same dollar, you'd get "more tomorrow, and more the day after that, and so on forever?" So, deflation by itself isn't going to render something useless as a currency, unless that deflation massively outpaces the interest you can get from a dollar-based investment, in which case investors will hoard the coins.
Lack of price stability will the primary deterrent to pricing anything in bitcoins.
The bank is almost certainly paying you less in interest then you're losing due to inflation. You're slowing the loss down, but you're not stopping it. Even US savings bonds will basically only break even with inflation.
Meanwhile, investing dollars in something else that does beat inflation (like the stock market) is risky. Sure, you can take steps to make it a lot safer but the money you make always comes with some risk of loss (ask anyone who lost their retirement funds in the recent economic troubles)
So if you've got both dollars and bitcoins, and you need to buy something (everyone has to eat), which are you going to spend? The dollars, which are slowly losing value and can only be invested with some risk... or the bitcoins which are always gaining value with absolutely no risk? It's going to be the dollars every single time.
As long as dollars exist, it will always be preferable to use them instead of the bitcoin, thus the bitcoin can never become a real currency instead of an investment. Your argument only works in bizzaro world where everyone uses bitcoin and has abandoned dollars.
(and on preview, alway has explained rather well why using bitcoin as the main form of currency would be bad for the country)