My math prof likened it to the axiomatic frameworks: if a person has bad facts or faulty reasoning, but your core axioms (values) are similar, you are more likely to come to a point of mutual agreement or at least understanding. If your values (axioms) are radically different, chances are your entire frameworks are different and possibly incompatible. You'll be talking past each other, discussing methods while in fact your goals and assumptions are completely different.
Philosophical answer - depends on your philosophy and how you feel about philosophical zombie - i.e. is it only outward results or internal causes that make a difference to you.
Although maybe that's only true if you find their values tolerable. Otherwise I suppose the advantage is in knowing you'll have to start strategizing if the conclusions from their values are incompatible with yours.
Either way it does seem valuable to know the other person's complete reasoning, just trying to quantify as many reasons why as possible.
It's pretty arrogant to assume there is only one way to look at things - your way.
If someone is acting malevolently, then you can rationalize basically anything with regards to hurting them or their well-being.
If someone just views the world differently, you will be annoyed by them, but probably not able to rationalize to yourself that they should not exist.
The first part is interesting, but ultimately, if their conclusions were based only on facts and reasoning, their conclusions would only be limited to the area of facts, "is" statements. They would not be able to derive ought/should statements. (If you believe in Hume's Guillotine.)