Consciously deciding on values gives you something to point to, however. You say it's wildly naive and unrealistic, and if you mean the act of writing them down is sufficient I agree; the point being made is that defining a team's goals is -necessary- (but not in and of itself sufficient).
Without making it a point to say "This is what we want to achieve", you arm yourself, your team, etc, with nothing to actually bring about the values you want to see upheld. Sure, you can just try to do them yourselves, but is the team in alignment? Do they agree with them? When you deviate, will they call you out on it? When they deviate, will you have the rest of the team's backing to bring things back into line?
The answer is no, if you haven't stated them explicitly and gotten buy in. The answer is possibly yes if you have. It still takes effort, mindfulness, etc, but it moves from the impossible to the possible by taking the time to define them with your team.
Sure, it's possible you hire so well, so perfectly, based on unstated values, that everyone is in alignment, all the time, 100%. My experience hasn't been that. I doubt most people's has.
To break into heavy metaphor, it's important to define your 'guiding star', as it were, so you can reorient the ship when someone points out you're adrift. Because without it, you not only can't tell you're adrift, you also have no idea what to reorient yourself to.