I agree with what you are saying in the context of taking on another's program. In similar situations, I have also attempted to rewrite the body of code. Once it was a necessity, as the code was perhaps 50% wishful thinking and never worked. I have tried in other cases, unwisely, and failed.

But when you are confronted with a pile of bad but working software, I have done what what you are saying here--dive in and understand what is happening.