Initially the CPU was being used (which is what you suggest), but this practice has ended. Now GPU is being used (do your servers even have graphic cards? Powerful ones?) because CPU processors do not hash correctly. Most (all?) software for mining is now GPU based and not CPU based. So your concept of combining a bunch of CPUs not otherwise in use will not work if this is as true as I believe it is. Next: Even GPU is about to become obsolete, as there are several hardware-based platforms poised to launch momentarily. These will use a teeny portion of CPU for the "handling app" and as such will not need "bunches of cpus" to manage them. In fact, the advertising for one manufacturer implies it is more efficient to use a USB hub to chain them together for more group processing power.