INTERNATIONAL and Russia were were important in Clinton and Goldin annual sale pitch to Congress. I'm not sure ISS could survive without at least symbolic Russia participation. Being "international" was essentially a ploy to make it cancellation-proof in Congress every year. Note that Canada / Europe are not enough: in 1993 even with them onboard, Freedom had a close brush to death, 216-217 !
Another teething issue of course is losing Soyuz. Since ESA is out of manned spaceflight and the Shuttle is doomed sooner rather than later...
Like it or not, but Russia, for all its flaws, is absolutely essential to ISS buildup - and later, somewhat by luck, Soyuz for crews since July 2011.
Depends whether STS-107 ends in disaster or not, but if that does not happen, Shuttle to 2020 or beyond is bound for a second accident at some point. Leaving the non-ISS without any crew vehicle, since the russians are out, and COTS / Ccdev never happened.
The most horrible thing with Shuttle accidents that left 14 astronauts dead, is that, in retrospect, were "necessary evils". I mean, they both stopped ongoing craziness by NASA - Challenger a little more than Columbia.
Challenger injected a huge dose of *realism* into the Shuttle program. No, the Shuttle can't fly 24 times a year, only 1rd of that: 8 times in 1996 is the realistic, max flight rate achievable without killing crews.
Columbia is more subtle: it put an end to the "Goldin era" that had some positive aspects but also broadly negative ones. That is, Goldin drive "faster better cheaper" was shamelessly used by the Clinton administration to cut NASA HSF budget by 20%. Kind of "hey, goldin said he would make NASA running cheaper, so we can happily slash its budget, without any remorse. The administrator endorsed the move."