Comment 2 for bug 769939

Just to show it very clearly, in case there is any doubt. I'll use gnome-terminal since it's semi transparent by default.

1) Open gnome-terminal and open a few tabs in it. You use alt+num to switch between tabs.
2) Press F11 to view gnome-terminal in fullscreen.
3) Press super and pretend you really wanted to press alt in order to switch between tabs, but hit the wrong key. This has happened to me quite a few times.
4) Observe that the dash is opened in the background and receives focus.

Even in this case, where you can actually see the dash pop up, it can cause a little bit of frustration. When the window isn't transparent, like Firefox or Totem, it becomes very confusing, and as Mr Silveira says, it's very easy to inadvertently start applications you didn't intend to run, specially Firefox. Then suddenly you have different instances of Firefox where tabs are placed "randomly" (it's easy for a user not to notice that a new instance has been started and therefore will receive new URLs from other applications, for instance) in different ones.

This clearly isn't the intended behaviour. The same goes for other lenses too, of course, like super+f and super+a. But since the dash is opened by a single press of super, it's much more annoying than with the other ones.