Performance isn't strictly bottlenecked by crypto operations. Latency is a huge problem with VPNs. This means you want packet-per-second(pps) performance to be optimal as well as full duplex packet processing.

Multi threaded means payload that has already been through a crypto operation wouldn't need to block further crypto ops or wait for other crypto ops to be processed further (just one obvious example of a performance advantage). Also - plenty of people don't even use AES,even for those that do AES-NI offloads the actual AES operation,not things like CBC,GCM and CTR.

That may be true for Intel CPUs, but you're forgetting the Raspberry Pis, ODROIDs, APUs et cetera.