A couple nice reasons for two CAT5e if they are going VoIP is that if you have to reset the phone (especially if the phone is PoE) you lose network connectivity briefly on the PC that is daisy chained to the phone. And if the phone's switch port dies or has issues, that adds to troubleshooting issues. It's easier to troubleshoot if the PC is directly connected to a port vs a phone.
Also, I haven't seen a lot of VoIP phones that have gigabit ethernet interfaces on them, if that matters to you. Those may exist however.
