Your question is, "Should they?"
It is clear they do not take showings into account. And they should not.
When ti comes to offers, we all know the lender is the third party negotiator. So, financed offers should be ignored as they are now.
Absorption rates and market trends are considered for scaling.
That leaves CASH offers.
Cash offers ARE THE AUTHORITATIVE voice and appraisers are not invited.
If the listing agent has a clue, he/she will know what the appraised value will be. The agent needs to know who the lender is, and kicked the squirrelly ones to the curb at the start.
Next step is to make the appraisers job easy. Now who doesn't prefer EASY?
Apprisals have not been a problem here in the Tampa Bay area of Florida, as long as Wells Fargo and Bank of America and Chase are kept out of the picture. Even when they are involved, the agent and the seller KNOW what to expect. Don't allow your seller to be ambushed.