Why is credibility a good thing?
For example, the US has credibility that it will defend Europe. What is the consequence of this? Europe slacking off on its own defense. This has socked the US with large expenses for decades.
I get the impression credibility is sugarcoating the idea that the US should do what others want, not what is in the best interests of the US.
There's a lot to be said for incredibility: doing things that no one expects or even believes possible.
Are you not asking two questions?
Credence is a good thing because others can understand websay what we do and do what we say.
This is high value! Expectations can be set and met with confidence. So much depends on that being solid!
Your other question, "Is the expectation the US will defend Europe a good thing?"
Frankly deserves and would be a good discussion I would read with interest.