It might not be in ARM's business model that it needs Apple to purchase them, and it would be unlikely to directly benefit ARM, however the benefits to Apple are huge.
- Being able to tailor designs to the requirements of their products
- Having processors optimised for iOS and OS X platforms (especially power management)
- Control over which competitors get what technology (although anti-trust would certainly be an issue)
- Optimise designs so that processor packaging (i.e. size and shape of the final product) suit the internal designs of their products where space is very limited (iPhones / iPod Touches)
- create high speed interconnect technologies between processor, graphics, memory, IO, etc
It would however be a surprising move by Apple, and as likely as Apple buying Foxconn.
It would also prevent Google buying them!