You might have noticed a trend that there are a lot of features in the next version of Hyper-V that increase uptime. Some of this is by avoiding unnecessary failovers. Some of this is by reducing the need to shutdown a VM/service in order to do engineering. This is one of the latter.
Most of the time, we only need single-homed (single vNIC) VMs. But there are times where we want to assign a VM to multiple networks. If we want to do this right now on WS2012 R2, we have to shut down the VM, add the NIC, and start it up again.
With Hyper-V vNext we will be able to hot-add and hot-remove vNICs and this much requested feature will save administrators some time and grief from service owners.
Better should be be HotAdd CPU in my opinion.