They absolutely are. And for what? One possible booking? If it takes you the ten minutes you’d waste showing potentials round to promote your listing, you could get ten bookings.
I usually reply saying that they are welcome to come to see the outside of the property (although why they should want to is beyond me) but they won’t be able to see the interior because it would be unfair to disturb the current guests. (The people who are inquiring wouldn’t like it if you were showing random strangers into ‘their’ place, after all).
There’s nothing to stop you doing it but I don’t see why a host would bother.
I also like to mention in my response that the listing and the photographs are 100% accurate. (Unspoken: "Don’t call me a bloody liar )