Im not sure that the real purpose of healthKit has been properly understood. I practice in the UK so dont follow US healthcare closely but from a recent international conference I attended, the signal seemed to be that Apple is looking at either becoming a healthcare provider or acquiring one.
HealthKit is about data acquisition for Apple. Not from the healthy, who take an active interest in their health but from the unhealthy that dont. Apple doesnt want to know that you ran 40km last week it wants to know that 30% of its users walked less than 500 steps/day (Im making all this data up) and, only because they bothered to fill it in, a BMI of 32.
Perhaps you also bothered to fill in the medication list and 15% of users are found to be on blood pressure medication.
All of this can be scaled up to population level and used for actuarial analysis.
If you do happen to already be part of a service that uses a form of tele-medicine then that could feed into HealthKit. I would see the next step as being HealthKit Server for physicians (sync the medicine lists, blood results etc.) which would be quite attractive to hospitals etc. if on going support were guaranteed followed by Apple reviewing the data prior to making some sort of larger investment.
As I say, Im UK based so make no judgement on whether this is a good thing or not.
Incidentally, I agree that recording exercise is meaningless without recording calorie input but I think its fair to assume that no one takes an active interest in logging a grossly unhealthy diet so exercise is a surrogate for health attitude.