I think commom courtsey is just something that should be with someone no matter what profession they are in. Also, there is that professional courtsey that should be a part of someone no matter what. I have 14 years in the business and the amount of real estate agents that respond when a prospect chooses something else is minimal. If I took the a generous amount of time to show you and your clients, if I call or text or email to ask what you client has decided I really do expect an answer, they chose another home, they are not in the market etc. It is just common and professional courtsey.
Many agents always want to build a relationship with the builders, this is an easy way to do it just respond it can't be that hard and you can't be that busy.