Thanks!
Yes, that's true, I was just wondering what you meant with primitives. For my side-scrolling game I normally use a different technique for transparent entities, I don't use entity.alpha. Instead, I render the entity in solid mode, suppress it in the default camera view and grab the pixel at the projected position. It's not "real" transparency but I like the effect (http://www.superku.de/sk_transparency.jpg) and no polygon issues are visible.