California Code calls for 16 or more units to have an onsite manager [Cal Code of Regulations, Title 24, 42} - which is why there are so many 15 unit buildings!!
My onsite managers are tenants with good boundaries and an even temper - because tenants can do crazy things in the middle of the night!. The manager gets reduced rent or even free rent in some of our larger projects.
Some of our clients even have onsite management along with our management because, while the onsite manager takes care of noise, repairs and other immediate issues, we are more aware of the current laws, accounting issues and legal/eviction issues.
Lastly, our onsite managers get an annual 1099 because they are "working" for that reduced rent.
Hope that helps!