An easy way to find the last error in the log file: open the error log in your favorite editor, go to the end, and search backward for 'error:'. You'll find it quickly.
Here's a scenario that you'll agree (I think) is bad:
You're working on putting some changes into the core (maybe you're even attempting to implement the change suggested by this TIP), and you make a mistake. The mistake you made just broke the APM, and it also affected the ability of the APM subsystem to load on startup.
What now? You do have a record of the initial error...in the error log, and you're not going to see the APM UI (actually, any UI) until you fix the problem.
How are you going to fix the error? You're going to use the error log, except now that's *all* you have.
If you had used the error log in the first place, you'd still have an operational system while fixing the typo. Just don't stop the service in this state, it will not start again.