... why would I use it as a transaction currency?
Inflation protection. Currently Bitcoin inflates rapidly, but the rate will dramatically reduce in some years, since it follows an asymptotic curve. Government currencies inflate
exponentially.
But yeah, that's more a "store of value" usage, not "means of transaction".
I understand that, as means of transaction, Bitcoin does not fit perfectly to every use cases, although it's great for some of them. And the reason it doesn't fit well to some use cases is mainly because conversion is still a hassle. Maybe one day it will be easier and cheaper to convert from and to legacy money. And with more business accepting bitcoins as payments, maybe one day it will be easier to earn your revenues in bitcoins as well, making conversions less necessary.