If the home is a million dollar listing than yes it may be appropriate for the agent to attend the buyer's agent showing. For under $500k, it's a little unreasonable and actually may injure the seller's chance of selling if his home is difficult to show because it has to coordinate with the listing agent's schedule.
Also the buyer's agent has a very good feel for exactly what his client is looking for for and has a level of comfort working with that buyer. I have had numerous uncomfortable showings where the Seller's agent was present during the showing. In over ten years I can say that perhaps 2 of the showings where the listing agent was present was actually beneficial. The other multiple occasions were not only very inconvenient to schedule the showings (I had to bypassshowing some because it was convenient for my buyers to see the property at the convenience of the listing agent) but also didn't allow my client to fully evaluate the house because they were completely uncomfortable.
Many studies have been done to prove that the listing agent (unless a million dollar plus type of listing) will actually hurt a seller if that seller demands the listing agent accompany all showings.
Thanks for the "best answer" votes, Gerard!
All the best,
For another agent to "show" my home to a perspective buyer, when he/she has "never" been in my home is ludicrous. How can he/she point out the qualities that make my home special when in fact, they've never seen my home until they show it for the first time...it's kind of like the blind leasding the blind.