It's actually the reason I started putting multiple types of standard weapons in my created warships. Most will now include missile, ballistic, and energy weapons, when previously I usually just shoved Large Pulse Lasers on it.
As for it's use, yes it is legal, and it was likely the most ideal armor on aerospace fighters shortly after it was introduced. But as other nations begin to see the deployment of large numbers of fighters with this armor, they're going to start working on doctrine to include counters to them. This is pretty easy with omnifighters, which are becoming more and more ubiquitous, where their weapon load outs can provide the flexibility to counter these fighters.