[RightWhale]

It has limited resources

The main limitation is the ability to recover everyone on the ISS at once in case of emergency. They could put up an extra 7 astronauts for a period of time.

[Lokibob]

Where do they get the o2, water, food?

If something happened on the ISS, how would 10 people fit into a 3 man rescue vehicle?

There are many things you and I have not considered. I was watching NASA TV the other day, and the ISS residents were putting on their EVA suits. The reason was, they were trying to do it by themselves, without help, in prep for the new caretaker mission. If one astronaut got sick, the other one would be forced to don his EVA suit alone, and NASA wanted to know if it could be done. This is just one example of the things you and I don't know or really think about.