I participated in 23andM3 research project too, mostly because my data point would add to PD knowledge, not for immediate personal benefit. I was also aware of the fact that they are doing just LRRK mutation analysis. I assumed that it is because the project is Google funded and LRRK mutations in PD are of interest to Google founder. Yes, it would have been a far better study had they included other genes too. May be they will at a later date.
Thanks for pointing out that absence of a genetic marker does not mean anything. Sadly, finding a mutation doesnot translate into a cure either. At this point, this project is mostly for science and data collection. It will help future generations, I hope.
girija