I expect that my case was similar to that of many others - I tried putting my code there, got a broken layout, tried figuring out what was wrong with my code (and got nowhere, as the code as was ok) and in the end just moved it to the lower ad template because it worked there and that was the quickest solution. Fast forward to a couple of weeks ago that I was working on another forum where we needed something up there and I see the same mess with another piece of code I put in. A few ticket responses later I now know why it's happening, but it's still bothering me for two reasons. First, the behaviour is not consistent with all the other ad templates (you shouldn't need to jump through hoops to make a piece of code dumped into an ad template show properly) and second, no warning anywhere in the ACP that this template behaves markedly differently from the others and how to handle it.
Ideally, I expect some kind of workaround could be used to make the layout automatically adjust to the inserted code's dimensions. But if not, at the least some pointers and/or warnings about it should be posted in the ACP, maybe commented in the template itself or whatever so that people not familiar with it know that they need to approach it differently and what to expect. As it is, I expect that most regular users wouldn't know how to make a block of code inserted in there show properly.
Sure, it's just one of a couple dozen of those XF things that you learn about through practice and remember for the future, but most new users encountering it for the first time will likely give up on it unnecessarily, assuming that their inserted code for some reason "doesn't work" in that template.