But it doesn't make a heck of a good unit of account, because it's too volatile. For a unit of account, you want something that a lender will be happy to be paid in, or a debtor happy to pay, when an account comes due in a month or a season or a year. (Note that "lender" includes the idea of "savings account holder" and "debtor" includes the idea of banks borrowing money by offering people savings accounts... )
To be happy about that, the lender *AND* the debtor would both need a much firmer idea about what the value of a bitcoin at that future date will be. Without that knowledge, you just don't get a common view of the world where the lender and the debtor can both be happy about their prospects of paying/getting paid when what they owe is measured in these units.
This argument has been made since about 2011. Give it time... it will stabilize over time, probably to somewhere between the actively-managed currencies and gold.
And it doesn't make a heck of a good store of value, for pretty much the same reasons. In a store of value, you want something that will allow you to get your value back out, undiminished, at pretty much any time, without worrying too much about fluctuating rates. The fact that people who "stored value" at $1100 last december only could get out about $600 today means that BTC is a failure as a store of value over that term, as compared to $USD. A good store of value is necessarily low-risk, and when we use BTC to store value we're exposing ourselves to a fairly substantial risk.
Tiresome. If you cherry-pick your dates you can do pretty terribly for fiat currencies, bonds, stocks, real estate, art... etc. Everything. Safest thing to do if you have near term commitments is to use the same unit of account. That is, if your loans are in Euro, price your salary in Euro.
And the idea that someone wants a stable value is BS. Nobody complains when their "stable" investment appreciates dramatically.
The fact is that there IS no good store of value, so if you really want to do this diversification is the only choice.