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.
The purpose of the ISS is to create work for the shuttles. The secondary mission for the ISS and the shuttles is to bring multiculturalism, diversity, and tolerance into space at great expense. An expense than now includes seven lives.
I came across a year old document on the web earlier today. Ironically it said that STS-107 was to have been the Columbia's last mission. The plan was to mothball it to save money. However, since then another mission had been added. Another teacher in space mission planned for later this year.