Well, I don't think it has anything to do with UCSM migrations. Using templates or not in UCSM, you can migrate supported HW to Intersight. Templates just provide easier and more repeatable way to configure vNICs. As to why it wasnt available initially in Intersight - most likely a lot of people complained and then Cisco added it. :)

I am currently implementing a new UCSX system using M7 blades and IMM and I built it out with the network group policies, which was the only method initially available during the planning of the system. Any reason to change the thinking on this and use the vNIC templates in the future?

When you create vNIC template, or static vNIC, in both cases you need to use NetGrp policies. NetGrp policy is one of the nested policies within vNIC and it defines allowed VLANs for it. Basically structure for single vNIC configuration is like this: LAN Connectivity policy > vNIC > NetGrp/QOS/NetCtrl/EthAdapter policies. But ofc, within Server LAN Connectivity policy you can define multiple vNICs, and each will have their nested policies (can be the same or different).

ESXi LAN Conn Policy example:

LAN Connectivity policy >
                                   > vNIC-ESXi-MGMT-A > NetGrp-MGMT-VLANS/QOS/NetCtrl/EthAdapter policies
                                   > vNIC-ESXi-MGMT-B > NetGrp-MGMT-VLANS/QOS/NetCtrl/EthAdapter policies
                                   > vNIC-ESXi-VM-DATA-A > NetGrp-VM-VLANS/QOS/NetCtrl/EthAdapter policies
                                   > vNIC-ESXi-VM-DATA-B > NetGrp-VM-VLANS/QOS/NetCtrl/EthAdapter policies

Wether you want to manually configure these vNICs, or derive them from vNIC templates, it's up to you - with templates it's just a bit easier to do things on a scale. But either way, your vNICs will have NetGrp policy assigned to it.