You don't need Agile, just let the developers do what they think is right?

Do we need this Agile malarky if all that really matters is letting developer do what they think is right?

This is a popular variant of "Agile is fundamentally just..." and is similar to "Agile is just for programmers", differing only in not wanting to identify with "Agile".

Let's work through the reasoning.

When building something effectively, how do you know what is right?

In order to know what is right, you need to understand both the problem space (What problem are we trying to solve? What forces are in play? etc.) and the solution space (What options do we have? What trade-offs are in play? etc.).

Nominally developers have the best understanding and insight into the solution space.  "Let the developers do what they think is right" implies that understanding the solution space magically means that you understand the problem space.  This doesn't make any logical sense.

One-way communication from developers to product owners / managers is as illogical as one-way communication from product owners / managers to developers.

Bringing perspectives together in order to gain understanding and insight of both problem and solution space does make logical sense.

Doing what you think is right is ineffective if you have no justifiable reason for those beliefs.

See also Lean Startup.