Like fiatjaff said, if you leak any of your privkeys they all get compromised to the relay who has the xpub. Probably not a good idea. I think maybe a reference to a list of pubkeys would be practical enough
Discussion
Leaking privkeys though… obviously this is bad in any scenario. In the context of nostr, I’m not sure what the specific concerns are with providing an xpub to a service provider. To me it seems equivalent to giving an xpub to a Bitcoin exchange, etc.
I’m not promoting the idea, just trying to think it could provide any UX benefits to nostr relays/services