It's not just about prep material quantity, it's about prep material quality.
I did it because it's cheaper than the Kaplan and TPR prep materials, and the answer keys are much better (really, I can't overstate this - looking at the answer keys should be as important to you as sitting down to take the test itself).
It's also harder. I don't really like the Kaplan and TPR practice tests. I looked at one after I took an AAMC MCAT and thought, "Wow, these aren't like the AAMC tests at all." I never felt that way with the GMAT tests. As I used the GMAT tests my AAMC test scores started to rise, which didn't happen after the kaplan tests. But to each their own.