--- The issue I take being that Boarding Capability makes units better than regular ones, and by extension means that while you can use regular INF for boarding and regular INF for defending, you HAVE to have the "better" stuff for anything else. However, without some way to restrict what can participate in the defense it runs into the problem of tanks driving on ships... which is admittedly weird. So a "Cannot" or "Do Not" participate button, perhaps requiring SM to toggle, would solve this without interfering with those who might want to RP that their ships have been built with room for tanks to drive around in them. Which is admittedly still weird, but highly amusing.
I think it makes plenty of sense, since it is Infantry OR Boarding Combat Capability. Any man with a gun can run down a corridor and start shooting at things (Infantry), but a large weapon system must be specialized for this - and of course any infantry unit specially trained for boarding combat whether on offense or defense should perform better in that situation. Aurora cannot cater to every RP, so maybe "we built big corridors so our tanks can drive through them" is not on the table, but I think for the majority of cases it makes sense.
--- I like being polite, but I'm gonna be a little rude here and say that's horsesmeg. Any idiot can deploy a Machine Gun and some sandbags, ala Light Armor Static w/ CAP, but someone with special training and equipment could do it better. With wide enough hallways any idiot could drive a tank into battle, but a specially designed one wouldn't need them and be more effective to boot. The problem is that Boarding capability does make it
possible it makes it
better. It's one thing to load up a ship with a bunch of machine gun toting yahoos and another thing entirely to load it up with specialized, purpose built turrets.
--- A SpaceMaster-Only toggle for Ground Forces that let's the player decide on a case by case basis whether or not their units are able to defend in a boarding action allows for this and more. Likewise an option for ships to have a toggle for allowing or disallowing Non-INF to participate in combat allows this and more, PLUS it allows the new Misc. Components to be used to fill that tangible design gap by accounting for the extra volume / mass needed for such things. Additionally, such toggles being SM-Only let's those involved in tournament play, or even just casual MP... if that's even a thing... to prohibit or control it's usage at will. Finally such toggles, which ought to be off by default and which ought to retain themselves after being switched on when SM is off, remove the need for a Special Rule to account for the extra functionality.
--- Instead of, "INF can do boarding AND defend boarding with or without Boarding Capability, but other units can defend BUT only WITH Boarding Capability." we get, 'Only INF can actually conduct a boarding, but anything can defend against it. However, if you don't much care for that, you can hit the SpaceMaster button and either disable it for ships, disable it per unit, or just disable it outright and make Non-INF units unable to participate in the defense." The latter only requires the player to interact with it if they don't like the rules as is nd leaves room for Boarding to be added to such for increased effectiveness or else omitted for cost concerns.
--- And to conclude, yet another approach would be to simply take the tonnage of the non-eligible units and treat it as an equal tonnage of "Crew" with PWL. However, these would need to be itemized so as to accurately represent the casualties of the individual units that they are meant to represent. The artillery guys getting slaughtered shouldn't wipe out the tank crews and vice versa. At any rate, mild apologies for the little bit of rudeness... but, I thought it was horsesmeg and I think it best to be honest about it.