at least according to this letter:

http://www.spaceref.com/news/viewsr.html?pid=10833

"Such experiments benefit in many important essential ways from being resident on the ISS. For example, the space station provides electrical power, spatial orientation, and telemetry to the experiments. These could be provided separately for reach experiment but only at considerable cost and duplication. For major experiments, location on the station offers substantial economy of scale, both in cost and in design and construction time."

"The ISS allows experimenters to focus on the scientific design and construction of complex experiments to achieve uncompromised results without the added cost and distraction of also building an independent spacecraft."

"Perhaps even more importantly, astronauts and cosmonauts on the ISS provide the possibility for human intervention to repair, modify or respond to unexpected developments. One need only think of the recent experience with the Hubble Space Telescope to appreciate the value of such human intervention. A major experiment involves not only capital costs in the range of billions of dollars, but also many years of development and hundreds, often thousands, of scientific man years. To launch such experiments without the possibility of repair and refurbishment is to accept a level of risk which would seriously impede the research in space, and which is likely to discourage scientists from inventing and working on some of the most important and exciting directions of space science."

I discussed this with Jack Sandweiss in person shortly after it was written, a couple of years ago; if you actually run the numbers, even with shuttle costs as they are, ISS makes real economic sense for such an experiment. I wish more people recognized this. There are certainly other worthy experiments of this sort with the same issues.