Ahhh they mean the per-application HUD. That is indeed tremendously useful, like the OS X searchable menus and Sublime's command palette. I can't see why anyone would be against that functionality.
If I had to guess: Because GNOME devs pursue their religious quest for simplicity ^W dumbing things down so far that they cannot acknowledge anymore that an application might have more than 5 menu items.
Wow, I wasn't aware that Unity had that! I mostly use Sublime because of this and whish all programs have one. Now I'm sad that it's gone…