I have a large ship design around $9000, that includes a few barracks, mainly as a defense against boarding. I guess they could be used to board as a backup, but they are preferred over guard barracks because they provide more guards per unit of space. The problem is that I give my ship to the AI to test things against it, since the AI aggressively pursues boarding at the expense of being able to use nearly all of its (many) guns, hazards itself by colliding with its opponent, and gets destroyed as a result of its stupidity.
The aggressiveness with which a ship holding any air marines at all needs to be toned down so that ships that are mostly dedicated to guns but have a few air marines thrown in don't pursue boarding over everything else - instead only boarding as an auxiliary tactic if the opportunity presents itself.
Either that, or make the guard barracks provide 4 guards (the graphic even has 4 bunks in it), trading the ability to board other ships for flexibility of space, and allowing people to use guards as a replacement for air marines when they don't want a ship to be used for boarding.
If needed, I can provide the affected ship designs. If I pit two AI-friendly versions where the barracks have been replaced with guard barracks the difference in behaviour is readily apparent. Having to make AI-friendly versions of ship designs just to accommodate this behaviour would be unideal, though.