I can see that argument working in a game like Project Reality, where they do have limited but strong single anti-vehicular options to manage strong but limited numbers of vehicles (at a time anyway); but, this is PlanetSide. Unless you're really going out of the way to inconvenience yourself, not many situations arise where you can't get some vehicle support. And, yet, they fit the situations you specified. It doesn't matter if they are slow - so are vanilla Infantry. Compared to proper Vehicles, the other two may as well both be moving at the same speed. Moreover, vanilla Infantry also need support - that's what makes them inherently strong enough to tackle anything. I'm not certain you should be trying to frame a hypothetical weapon around specific circumstances in this manner, like arranging dolls in a certain way. It's going to end up being used whenever someone can pull it so that should be the situation to which it matters. MAXes, before anything else, are Infantry-sized sub-Vehicles and will end up in the same circumstances as their squishier cousins.