Setup time doesn't stop it from being succesfully used as AA defence, especially behind the lines. The only reason it is not used in that role is the fact that AA is completely covered by buildings that every player builds anyway.
No seriously FlakHT is not build because its plain useless. There are tier-0 counters to it. Only Job it could do is shoot down planes and for that i would never use popcap, MP, fuel or Munitions to MAYBE (rly big maybe) shoot down strafing plane i could just easily dodge.
Yeah there are scout planes... But then again there are flares i cant shoot down those either...
Remove the Schwere HQ altogether make it an Tier-upgrade on the first truck. Buff AA-Ht from useless to equal to USF AA-Ht level. And let the OKW build AA encamplacements equal to bofors.