As someone who trains a lot for bike races, running races and triathlons. I use HeartWatch primarily to track sleeping heart rate, resting pulse, etc. to spot signs of overtraining, impending illness (colds, etc.) and overall sleep quality.
For me, I appreciate all of the different views and slices of data.
I don't however use it for exercise tracking specifically, even though it does show a great view of heart rate for any exercise that is recorded by the watch.
Personally, I find the app very useful and have never found it to drain my phone battery. Looking at the last 7 days of battery usage on my iPhone 6+, it only shows 1% for HeartWatch vs. the native Apple Watch app at 8%.