The trouble is even if this fairly unlikely scenario of large numbers of banks starting to using Ripple came about, there is no direct correlation between the use of Ripple IOUs for things like USD transfers and XRP having any significant value.

For XRP to justify even its current price let along a significant rise, it would need to be used as a market making currency for lots of trades (eg Euro > XRP > USD) but I really don't see that happening when people can just create direct exchange markets (eg Euro > USD) and reduce their costs and risks.