ARAM is a completely different environment. Nobody cares how powerful Mark/Dash would be in SR. Mark/Dash isn't in SR. The thing that would make Mark/Dash so strong is the fact that it would open most of the jungle as a gank path.
ARAM has no gank paths, because it has no jungle to gank from. There are no other lanes. There is no river. ARAM has none of the things that would make Mark/Dash broken. On ARAM, you will always see it coming, and always have a general idea of the enemy's location (dead, in lane, or on their way to lane). If they're alive and not visible, they are always either in front of you somewhere, or behind you somewhere (in which case you will know about it, because you saw them dive that far). That means you always know what direction the engage is coming from, and you always have an opportunity to avoid it.
Imagine having total vision of all possible gank paths, or even just knowing the path that will be used for the next gank. Easy to avoid getting ganked, now, isn't it? Perhaps more accurately, imagine that junglers could only gank by coming up the lane - the single worst way to gank on SR for anyone not named Sion. In ARAM, the only gank path is the lane. The only PATH is the lane. Mark/Dash isn't broken on ARAM, because the elements that would make it broken aren't on ARAM.
Your entire argument seems (to me) to be that Mark/Dash provides a little extra poke to poke champions (that pales in comparison to what some champions already have in their kits), and that it would be overpowered on SR (which, I will say again, is not ARAM, and therefore is completely irrelevant). Your argument is completely invalid.