Stable assets are more useful for writing long-term contracts. People who are trying to avoid risk (rather than pursue it) will want to write contract terms that have future payments in a stable currency rather than in some more volatile asset like gold or stock. For example, if you wrote a contract where one person does the work and the other one pays in gold in five years, how do you know whether it's fair or not? Would you be willing to take either side of the bet? It's all going to depend on where you think gold will be, and if opinions differ on that then it makes contracts that much harder to write.

Of course there is inflation risk with any currency, but usually it's at least possible to estimate it and make allowances for it when setting a future price. It's a matter of degree and Bitcoin isn't even close to being stable. Both upside and downside risk are a problem, for opposite sides of any deal.

There's also little reason to think that broader adoption will make it more stable. Supply is fixed, but demand could go either way, depending on greed and fear. For a stable currency, you need a stable demand that can withstand (at least to some extent) big swings in popular sentiment.