That it's predominantly used for speculation is the reason it has had high volatility, which is the point. Increased use as a currency would cause the volatility to decline, because when the majority of the value is held by a hundred million people each holding three-digit numbers of dollars worth, they don't try to predict the market and dump their holding of pocket money. So volatility not only doesn't preclude use as a currency, wider use as a currency solves the volatility.

There is also a pretty obvious way to avoid the volatility in the interim when using it as a currency: Don't use it to specify prices and don't hoard large quantities of it. If you have a Bitcoin wallet with the equivalent of $100 in it, it doesn't matter that much if it goes down to $50 and then up to $150 and then back to $100 again over the course of a year, because +/- $50/year is not a big deal. Meanwhile we have computers now, so prices can be listed in US dollars or any other currency and then use the live exchange rate when paying in cryptocurrency, while still accepting it widely.

And "it has no other purpose" is a weird claim. It has an obvious purpose: It allows you to exchange value without permission or identity. The "it's a public ledger so there's no privacy" claim is silly; your wallet address is public but you can have arbitrarily many of them, there is nothing inherently tying them to your identity, and there are known implementations (e.g. Monero) that provide even stronger privacy. These are things the existing banking system doesn't provide, and from the perspective of countries that actually respect the privacy of their citizens (or any citizens who want their privacy protected), are features.