the point of the topmid amoke is that it allows the ts to cross to the boxes and complicates the round by allowing them to peek from the other angles.
of course there are some plays where you can swing through the smoke onto cat or swing valley with a flash as well, but at the basic level the smoke exists to deny the cts information as to what the ts are doing in mid, so they have to play with the possibility that the ts \*can cross to mid boxes.
so if the ts arent throwing it, it makes your job in mid a lot easier as a ct because you only have to pay attention to underpass and the one angle that the ts can be peeking in top of mid. this possibility of the ts being behind the boxes does not exist as long as you have eyes on the top of mid without this smoke.this makes things like awping con or on top of the bench at the start of the round quite strong. if you can take mid as a ct you get to stack bombsites based on info that you have. remember, the goal of the cts on mirage is to either take mid or a main, if you are taking mid, then you get to eliminate the possibility of splits using connector or mid, which makes the ts job a LOT harder.