Are these ads for your apps or ads for other people’s apps?
If they are for your apps, then a low CPI could indicate marketing changes needed on your side. i.e. screen shots, descriptions. You still have to sell your app to the user. All the ad did was make them aware of your product and the ad creative was enough to get them curious, but you still have to close the deal. App/Game quality, art quality, functionality and price also impact them hitting that final button.
If these are for other people’s apps and products, you probably should be interested in CPC campaigns so that you get paid by the click. You’re generating clicks which is a good thing that means your app handles the ads in a way that the user feels they can click the ads and learn more about the product being advertised. But if these ads are for other peoples things and you’re being paid per click not per install, then the world is happy. CPI is dangerous because you could get served compelling ads to app that are not going to get installed because of poor screen shots, descriptions, functionality and price.
I don’t use RevMob so I don’t know how they handle their advertising. The comments above are generalizations.
Rob