It's quite an old ISE feature now, and not much discussion around it. If I recall from the stuff I have read, the feature is optional and it won't hurt NOT implementing it. However, if you operate a guest portal on two PSNs, then it makes sense to implement Node Groups, because the session persistence will be across both PSNs - if an endpoint is redirected to PSN1 but for some reason the redirection doesn't work, then there is some kind of failover (CoA I think ... I can't remember the details) to cause the endpoint to get re-directed to PSN2, which has the same state information for that guest redirection. To the end user it should look seamless.
Cisco goes into technical details about JGroups etc. - in my opinion, these are developer optimisations that I don't understand, and I always hope that I am doing the right thing by putting PSNs (which are in the same data center ... not necessarily the same VLAN) will have some benefit. How will we know for sure? No idea. I have not bothered to look into the details again of why I should use Node Groups. I just do it. It doesn't seem to make anything worse. Don't implement Node Groups on PSNs that are connected over a WAN.
If anyone has any pragmatic advice around Node Groups, I'd love to hear it.
