It is ethical as long as it is disclosed on the purchase offer. If the property is similar to a lot of other ones, this is one way to differentiate it from the competition. It's going to be an incentive to an agent to include that property to be shown to a buyer, when it might otherwise be overlooked.
Yes, I do feel it is an ethical practice and I have offered it on some of my listings. Does it work? It didn't on mine; but every case is different. I feel it really goes back to what the buyer is looking for and wanting in a home. I still would try to offer it again if my seller approved it and felt it would be beneficial. I hope this helped.