The reason is still that fallback mode is not maintained. Giving people the ability to make it a GNOME 2 experience was a nice benefit. Meaning: I didn't know people even wanted to make such contributions until the gnome-panel maintainer brought it up!
Unfortunately, it seems that various other projects have specific support to make fallback mode work. So initially I was happy that as a side-benefit people would be able to get a GNOME 2 experience. At the moment I'm doubting if it is feasible because it involves way more than just gnome-panel and metacity.
BTW: Fallback mode has been discussed during the development of 3.4 already. Only since 2-3 weeks I heard that Unity apparently depends on various fallback mode bits.
So sometimes things are not intended in a bad way, but they can easily be interpreted as such. E.g. "GNOME fallback removal to kill Unity".. while in practice we (at least me) had no idea despite having discussed this for 6+ months. For the Unity case we did ask Canonical before doing an announcement... though we work in the open so official announcement is often way later than sites reporting it as news.