I think the one big advantage of having their own chips is the marketing angle. It's much cooler to say an Apple iPhone comes with an Apple A5 chip than with a Samsung Exynos chip.
To be more than anecdotal, the iPhone 4S site lists the "Dual-core A5 processor" as both the first bullet item, and later as its own slide.
Surprising given that tech specs are said to be obsolete. But I think it's more about the dual-core, and it doesn't mention that it's an Apple processor.
Samsung is pretty explicit about what their SoC contains: http://www.samsung.com/global/business/semiconductor/product... - well they have to, opposed to Apple they're selling the chip seperately.