The first answer is that we're doing this largely because, well, we didn't know CCMS was a predictive model. Fools stomp in where angels won't even fly over for reconnaissance, and all that. What we did know was that it was open source, and while our chances of doing meaningful science with this are vanishingly small, we can document the transparency of the process. Again, the narrative of our efforts is one of "We Do Dumb Things So Others Can See Our Mistakes And Avoid Them."
The second point, it's the contention of most of us that CRU has significantly damaged its credibility. If similar behavior had occurred in the financial sector, people would be facing criminal charges. In scientific circles, they've put a dent on their reputation...and we can't get their data sources to validate from, yet. If we can, we'll expand the scope of what we're trying. In the mean time, we'll work with what we can get.
Because CCMS is a predictive model, we can do a couple of sideband tests - like see if we can get it to replicate, say, 1900 to 1940 and match the temperature data as a 'stable CO2 emissions epoch' with reasonable temperature data. Or tweak a known variable in a known way and see if it gets a result that makes some sort of sense as a first approximation at falsifiability.
We still, as mentioned, may not be able to get this up and running at all. If we can, we have, for lack of a better term, an exploratory toy to poke around with. We really aren't trained to do substantiative climate science with it; if we're very lucky, we manage to help the people building this model identify some undocumented assumptions and variables, and open discussion on forcing factors.