TWT, I am all for it, and in fact, have thought about building one on my property. I could not afford to build it and make money on it by renting it, but I believe the affordable rent could make it a break even proposition while helping another person, and adding a little excitement to my life. I am readying to build a new home in the near future, but will probably hold off on the Carriage House for maybe another year. If money were not an issue, I would do build it today.
Truly, having a long term renter's "servants quarters" could be handy for those who needed a cleaning person, ie- servant, to clean between guests and owners' visits in exchange for rent. You could also have this person maintain a garden on the property, or stock your groceries for the arrival, check in renters to the main house, etc. Why people would rather pay more to a rental management company than having a reliable tenant take care of things is beyond me.