You took "reflective" too literally. I just meant more reflective to IR than dark green paint that most military stuff gets painted with. I never meant a mirror surface. Just the IR equivalent of white paint.
White paint (or IR-colored paint, in this case) is worse than a mirror surface; it starts out at a disadvantage already. By assuming a mirror surface, I'm giving you the benefit of the doubt- anything else would be even more useless.
Are you saying that picking a coating with low absorption in the part of the spectrum that the laser is in doesn't matter?
I'm not sure I agree with you, since it makes other strategies (such as rolling the projectile in flight or using ablative materials) more effective if the projectile is slower to absorb energy from the laser.
The whole name of the game is to collect as little energy as possible, disperse it as evenly as possible (in non-important areas), and avoid deep penetration.
It doesn't magically stop the laser to paint it the right color, but choosing the right paint is an important part to penetrating laser based defenses.
For a sufficiently powerful laser weapon, yes, low absorption doesn't matter.
If the attacker is just trying to heat up the bulk of the target until it melts, or is otherwise disabled, then a low-absorption coating would indeed slow them down. Such weapons have been proposed, but they're not especially effective anyway, and that's not how modern experimental military lasers are designed to work.
Weapons lasers are designed to focus as much power as possible into as small an area as possible in order to cause physical damage at the point of impact- blasting a hole in the thing, setting it on fire, etc. Heating the bulk of the target is a mere side-effect.
In that regime, anything less than 100% reflectivity is essentially pointless. Sure, it'll delay penetration, but not by any significant amount. Within a very tiny fraction of a second, the part of the laser energy that you do absorb in the target spot will be enough to destroy the reflective coating, and bring the full beam energy to bear. Unless your projectiles are expected to have flight times measured in milliseconds, the delay provided by a special low-absorption coating is utterly useless.
If you can prevent the enemy from dwelling on a particular spot, then things look much better. If you can screw up their targeting quickly enough, maybe you can actually prevent damage to the reflective coating after all. Even if you can't, maybe you can keep the laser occupied with constantly eating through fresh areas of reflective armor, never absorbing the beam's full power. But at that point, it's not the low-absorption coating that's saving you- it's your ability to screw up their targeting.
Again, I don't think reflective coatings are a lone solution, but I do think that they enhance other tactics, and as such, are a key aspect of defense in depth. However, those contributions are key to those other systems working at all.
For example, the typical bullet rotates at ~200,000RPM. This is about 3,000 revolutions a second, or about 300 microseconds a revolution. Figuring a larger projectile could only rotate about a tenth of the rate, we get about 3 milliseconds per rotation on a shell.
At 3 seconds per revolution, I suspect that we're inside of the timing windows that talking about the efficiency of energy absorption and dissipation in the coating that the laser is striking is relevant, and we can begin to talk about the laser energy being dissipated over more than a single fixed point.
However, I suspect that such tactics (as high velocity spinning) can only mitigate the effects of the lasers and not stop them in isolation. It needs to be tied together with a coherent plan to dissipate the energy that areas do absorb and scatter the beam's focus using ablated coating.
In terms of energy absorption rates, energy dissipation rates, and ablative properties, picking the right coating for your mortars trying to pierce laser defenses is essential.
tl;dr: It's a synergy thing, since the low-absorption coating reduces the demands on screwing up their targeting, and screwing up their targeting reduces the demand on a protective coating.