Well, I might care about predicting the next turbine to fail. If Siemens sensors are truly unrelated to the issues, that'll average out eventually - but I'd be highly skeptical of someone asserting that it's completely unrelated to the failures and not just covarying with something we're not using as a model input.
Why would I care about the fact that only 10% of turbines globally have Siemens sensors? I don't know the failure data outside of the turbines I own and operate, and those are the only ones I need to predict failures for.