This is great for defense. Unlike some people have said, you don't need to physically destroy the entire missile engaging you. For IR heat-seekers, you just have to blind the seeker. For radar guided missiles, deform the radome. Missiles tend to travel at high speeds, if you can screw up the radome or any part of the structure sufficiently it'll make a big difference to the attacking missiles pk (probability of a kill).
Your same offensive weapon makes an awesome countermeasure against HOBS (High-Off-Boresight) stuff that someone might launch using a HMCS (Helmet Mounted Cueing System). If you have decent secure networking, there's no reason why a bunch of your team couldn't target the same target too. So instead of being hit by one laser, you hit the target with N lasers. The enemy having better kinematics becomes moot too. A rotating mirror can rotate much faster than even the most maneuverable airframe.
The best countermeasure to this stuff if you don't have equivalent stealth? It's tough. You can't detect attackers well enough to get a firing-solution, you have nothing on your warning receivers for your team. Best case, let's say you know somethings up there due to VHF radar. So you send up your stuff, and all of them just get swatted from the sky. You ask your best engineers what to do about it, and they say 'Our best idea is to make the environment so nasty we deny the enemy access'. How do you do that? Nuke your own airspace. If you can't see the enemy but your assets start exploding, fire off a pile of SAMS (in nice solid reflective casings, no fine guidance necessary) and nuke your own the airspace.
If they are at altitude then that's one thing (not much fallout). If they are using terrain shadowing / strike teams going in to take out your ground assets, then you are talking about basically carpeting yourself with fallout.