User blog comment:SilverHexxitFights/Which is Terraria's Hardest Boss?/@comment-24151017-20131111054633/@comment-1466854-20131111215836

Just because something can be done doesn't mean it was intended. They've accepted that people have found a way to do it, but that doesn't change the intent of the mob. There are other factors as well, like the fact that he's unsummonable once you beat Skeletron the first time (in the same world at least).