For (ii), see the debates about PSA tests. You may diagnose potential for cancer, but if the treatment causes more health problems than the disease; you've just created useless worry and costs with your test. The optimists see two things (1) better tests that could tell whether the prostate cancer is problematic and (2) as lifespans increase, the likelihood of you living long enough for the cancer to be a problem increases, so if we are trying to live forever, then the PSA test results will matter eventually.
Back to (i), the false positives. As a disease becomes more rare, the more your false positive rate matters. If a disease affects 1 in 10k people, your 0.1% FPR will diagnose 10 times as many people wrongly as it does correctly. This is pretty damn common with genetic tests.
I think you can get somewhere data mining whole genome sequences, particularly if you sequence from healthy and diseased tissues in individuals and do exome sequencing as well, but that doesn't appear to be what Color Genomics actually does.
Far more interesting is the data the Color will collect if they grow a customer base, which is not something that many researchers have access to. However, if they're only collecting features and not the labels (cancer incidence) then it's pretty boring data.
I don't think that it's a good idea if we end up in the scenario you just described, but what if they find something useful? I think that putting money into cancer research, no matter on what approach, is usually a good idea.
A lot of breakthroughs in history of science came from unrelated areas of research such as this where computer people are working in (at first) non-computer field.