The main advantage of ARM over Intel is power-efficiency. So, a rumour about ARM-powered iMacs at first sounds stupid and unnecessary. However, if Apple is bringing ARM to the MacBooks, it seems silly to fragment the Mac lineup by processor architecture, with some Macs running on Intel and some Macs running on ARM.
The benefits of switching to ARM will be realised in Apple’s laptop business, bringing incredibly high power-per-watt characteristics, which makes up more than half of all Mac sales. In contrast, I do not believe ARM is superior to x86 in high-performance setups. Because of the MacBook though, I think it is inevitable that the iMac and Mac mini will be transitioned to ARM as well, for the sake of platform uniformity. The iMac may even get marginally slower in the changeover.
Having everything running on the same architecture is good for the OS X platform. A move to ARM is fantastic for the mobile Macs, but is simply an implementation detail for the iMac and Mac mini, as a by-product of the ARM MacBooks.