If Ripple gets full market saturation as a transaction protocol, the value of actual XRP becomes relatively meaningless - because it would mean all institutions are connected via trust. It would only need to cost just enough to have some value to the transaction fee.
This wouldn't ever happen though. Just like not everyone on the planet is using an iPhone. Some have Androids. Some have Blackberries? And some don't use phones at all.
Bank A trusts Bank B, so they just burn a single XRP droplet for transactions between each other. However, Bank A needs $100 from Bank C - who they don't trust. But that's OK, because Bank B trusts Bank C. So Bank C sends $100 to Bank A (but it gets routed through Bank B to Bank A) all on a single XRP droplet. One day Bank A needs to send $50 to Bank D. Bank D doesn't trust Bank A. And Bank D doesn't know any of Bank A's friends. Because there is no inherent trust, Bank A has to send $50 worth of XRP direct to Bank D, instead of just an IOU. The Ripple ledgers at Bank A, Bank B, Bank C and Bank D (and all other banks) confirm and agree on this transaction through the Ripple protocol - so the money exchange can be trusted. Now Bank D can be certain they now have $50 from Bank A, while still never having to actually trust Bank A. And the cost to send that $50 was still only a single XRP droplet - except in the final case - an actual digital currency (XRP) was transferred from one account to another. Bank D can either sell this XRP for fiat $50, or keep for future transactions.
If that XRP loses its market value and is now only worth $40 in their fiat - then Bank D is ******* ****** and never uses Ripple again. So - it's in the Ripple co's very best interest to keep the value of XRP stable or forever increasing. This is a benefit of centralization.
In the above example, Bank A to Bank D is why banks will need to own some XRP and will require more if they run out. Whether it's given to them for free by Ripple or they have to purchase it in bulk - I am not sure. The more banks that adopt Ripple, the more "decentralized" it becomes and the less they need to exchange actual XRP. It's still not a truly decentralized network, but it's far better than what banks use today - which is completely centralized.
Paper math says the value of XRP always has to be above a meaningful transaction fee, and potentially increase as XRP are destroyed through transactions. I think the final value of XRP will be what the service and fee are worth to the banks. And it has to be less than what they are paying now, for similar, yet slower transactions that rely on a centralized entity of failure.