Your point about the points cost of Sky Fortresses is a good one, and one that makes no logical sense to me unless the theoretical opportunity for true Sky Dreadnoughts was being left open intentionally - guess we'll never know.
I don't play at 800 points, for the reasons above - games that are too small remove tactical ability as a determining factor.
In general, if I'm playing against a Sky Fortress, the aim will be, depending on the rest of their force, either to damage and then ignore it in favour of winning by killing other stuff, or it will be to bring it down asap (preferably by prize-ing) if they have too few other scary elements to worry about, or too many resistant to damage from range perhaps.
Either way, I rarely don't have an answer, largely because I play with Volley Guns.