I you make a very good point. In the past I would let known broken stuff ride while trying to fix other problems of more importance. But now with vehicles systems being so interrelated, I go out of my way to fix anything that aint workin right in an effort to get to the root cause of a concern.
2 examples: had a focus that would go into pcm reset mode when you would turn on the accessories. It was just like you cycle the key off and then on while driving. Upon visual inspection noticed that the motor had been replaced by the biggest hack whom I have yet to meet. Bolts missing, lines routed wrong, a/c manifold not even hooked up just dangling, ect. Well after frickin with this thing for a week and even and fse looking at it (who gave up on it by the way), I decided to just put the car back together as it should be. After doing so the car was fixed for it's concern. It turned out that the alternator power wire that is supposed to run over the top of the valve cover was run under the oil pan and zip tied to the crank sensor harness. Talk about some serious rfi.
The other was a 95 powerstroke with a bad pcm (chipped and fried). While getting to that found a few other problems including a seriously screwed up glow plug relay (had a mind of its own). Had a hard time getting the customer to put a pcm in it. Replaced the pcm only and got it running then it stalled then it wouldn't run again. Turns out the glow plug relay was zapping the system such as a bad ebp would with 5 volt ref.