Wheezie,
I do believe people inhaling secondary Tobi and Colistin can build up resistance to these antibiotics particularly in their sinuses, especially if they have any kind of active minor sinus infection. The argument that they are only getting a small amount is why I think this would be an issue, because they are not getting enough to kill anything, but enough for the bacteria to build up resistance. Much in the same way some of the vaccines we get for viruses. A small amount of the virus is given but not enough to give us the virus, just enough so we can build up resistance to it.
Also, some hospitals now require a hepa filter air purifier in your hospital room when you are doing Tobi so that no/less Tobi gets circulated through the air ducts. If you always use a pari lc plus type neb cup you are helping things since it emits less drug into the air. If you use one of the disposible ones you are emiting much more into the air for other people to breath.