Let's get a few things straight. Banks partnering with Ripple Labs to use their protocol does not mean that they use Ripple as a currency to power transactions. It simply means that they use Ripple's protocol to manage ledgers.

Also Ripple Labs owns the bulk of the Ripple supply. They are floating 1 billion Ripple coins in the market each month (There's no mining). Unless there's demand for those freshly launched coins in the market, the price will rise and dip on a cyclic basis.