I've been on the side of advocating for it, but the other side isn't making an obviously ridiculous argument. The onion protocol itself negotiates an end-to-end cryptographic session, authenticated by the onion site's public key, between the onion site and the end user. There's not cleartext traffic sent between a Tor exit node and the onion site or anything!

My argument in favor of TLS to onion sites was that, at least as of onion protocol v2, the cryptographic session was not itself TLS, and its security and threat model hadn't been as extensively reviewed as those of TLS. So it might turn out that there was something suboptimal there that the rendezvous server could then use to perform a sophisticated cryptographic attack.

I don't know if this is still true of onion protocol v3 or if the client-to-onion-site session is now also based on TLS.