Health Apps on Phones?

This post is about trustworthiness (security in a broad sense) and specifically about reliability.

I see increasingly frequent suggestions that people should use their phones to monitor their health. This is, on the face of it, attractive; being an insulin-dependent diabetic, I carry a blood glucose meter with me pretty much everywhere, and in line with the general trend of convergence (calculator, camera, music player, radio, etc.) wouldn’t it be great if that was built in to my phone?

Well yes, that would be very convenient, but I’m afraid I think it’s a fundamentally bad idea.