All the OSM iPhone apps that I have seen use the rendered tiles rather than then base vector data.

I suspect this is because all the various map rendering code that is out there to use is GPL and so they can't just use that to render the maps on the device. And it would take someone a lot of time and effort to create their own (half decent) map renderer. It is a very hard thing to get right.

Whereas they can use one of the GPL map back ends to render to tiles very easily.

Also, the OSM data, even in base vector form is huge, like 170GB huge.