Given the style of the book, it is impossible to say whether Naming and Necessity's main thesis is true or false. The most appropriate truth value is incomplete. However, the truth value is not the exciting part of the book. It is the possible that science is metaphysically necessary. If we take names to be rigid designates and the names are contingently identical, then they are necessarily identical. Kripke's example is "Heat is the motion of molecules". Once heat is identified with the motion of molecules, then they most be identical in all possible worlds. This understanding of names (if correct) shields science from most forms of skepticism. The only form of skepticism left is the mundane possibility of miscalculating.
Kripke's thesis is grandiose in its implications and plausible, these two qualities make it valuable. Even though it is an important thesis, I think Kripke's understanding of identity is flawed. He uses his thesis to justify "a cat is an animal" has a necessary truth. However, identity is reflexive and an animal is a cat is not necessarily true. So, Kripke's identity must be an implication of sorts. Or Kripke's identity only holds for a subgroup of rigid designates.