Skybasing isn't bannable on most uhc servers. Skybasing is okay until meetup were people have to go and fight. Now I'm not saying this should be removed from the rules but I generally think skybasing is okay as long as it doesn't correspond to meetup.
The reason why we have skybasing disabled on this server, is because that creates an unfair advantage. I myself, brought this question up recently just for some clarification within the staff team and the "unfair advantage" makes complete sense and I wouldn't want to be put in that situation especially if I want to PVP, and gain some PVP points.
Having it enabled at just meetup is a cool suggestion, however I feel this could create some confusion again and it could make more people ignore the specifics and skybase during the game.