Banks don't need to use XRP they can just work with Ripple
But Ripple has plan for adoption
- Banks can use their Ripple Net software to send fiat money without using XRP,
That doesn't change the way banks do things today.
What also comes with Ripple net is feature called xRapid, and that is part 2 of the plan - Banks will have the 'options' of using XRP or not; but they will want to eventually;
They will save an additional 30% on top of 30% that they already saving using Ripplenet on transfer fees
Who does not want to make more and be better the competition :)