What are the implications of leaving the XHC20 interface "up"?
Yes, you can leave the interface "up" when not actively sniffing it.

The interface is actually a sort of pseudo interface based on BPF. Those are generally very performant, and have little to none implication for performance and memory usage.

In terms of security, you could be exposing the your USB traffic to other users on the same Mac that have access to the interface, but not otherwise to your USB traffic. This is not commonly the case on macOS installations in homes / offices.