Braun himself admitted that the GECK was very unreliable, and I believe James knew this, too. If he were to use it to try and turn the wasteland into a lush, green paradise and it didn't work, the GECK would have been wasted, and they would have made no progress.
On the other hand, taking a few parts from it and using them to power the Purifier is simple, and will probably be more helpful and affect a larger area.
True. I just hope he kept those holodisks, re-educating the wasteland is the first step to rebuilding civilization. :vaultboy:
Also, I have a suspicion it might have been the cold fusion/fission generator he stole off the GECK, I mean, it's likely something far beyond the capabilities of a 4-5 wasteland scientists to reproduce. and it sounds like it could be powerful enough to power the Purifier.