There's nothing wrong with R-slot AA. It's not ideal if planes are running circles around your ship, but avoiding that situation as best as you can,seperates a decent AA player from the spamwhores.
Most R-slot AA has 1 big advantage over most T-slot AA, the KM40 and far more ammo ( = more xp/cash in more AA intense/lenghty games).
The 1 big problem with Emden is displacement. If you use 4 gold bar AA gunners, you have hardly any displacement left for anything else.