You'd have to keep an eye on their family visitors, but that seems to be pretty manageable, from my experience in two of these buildings now.

There is a difference between a overnight guest and a extended visitor.

They do it and I never have to visit or interact with the tenants.  

It's when they are not neutered that the male cats spray that disgusting whatever-it-is all over the place.We also mention new pets and visitors pets and fines for each.  

Now I just need to figure out how to spell out in the lease/addendum that no guests, visitors, etc. will be allowed to smoke on the premises. 

We pay $70 to have the apartment thoroughly cleaned between each visitor and we add this fee to the reservation total.

Now we just have to try and keep the visitors from coming back at night.

When I asked about her (we thought she was over 18), because male tenant said "we are thinking of having her stay with us for the summer" (violation of lease which says visitors can stay 7 days), the female tenant replied "he misled you, sister visits for help with homework, she spends the night sometimes".  

I mean, a drug dealer will always have enough money to pay the rent...and if he can't surely all of the visitors he'll have will convince him to do the right thing, even if they are all parking on the street and the neighbors are calling the cops...."

People who rent their residences to visitors are displacing hotel stays and cities are losing that tax revenue.