Excellent article on app performance and what you can do about it: https://medium.com/@mandrigin/ios-app-performance-instruments-beyond-48fe7b7cdf2#.dkuyz02mr
Measuring the startup time is really easy and can have a great impact on the user experience. I wouldn’t release an app with an extra thread to detect responsiveness issues on the main thread, but it is definitely a good idea in testing environment. It can point out some issues at an early stage. Collecting performance metrics from your users also seems a good idea to me.
Note: I didn’t mention anything about Instruments, because it is obvious you should use it regularly.