Well no one on the ARM market other then AMD has the expertise in Server CPU. Nvidia may be able to do a very High Performance ARM CPU for HPC but it is completely different matter on Server. There will also be NAS, Small Server, etc and whole not of other market where a big name ARM player could help. Although most of that wont happen till 2015. I am not sure if there is enough time for AMD.

And given the recent sales figures that 70-80% of CPU sales are coming from APU, the APU is rasiing, CPU is a shrinking business, it makes sense for AMD to consolidate and make that 80% better and slightly more profitable rather then trying to complete with an aggressive Intel.