OKW Flak truck is a nice deterrent but it costs 120 precious fuel to build and is critical for bringing in tanks. Losing it hurts way more than losing a stupid BOFORS emplacement.
I'll only put a flak truck on a critical point if we are crushing the other team and I just want the truck to babysit the point while I do other stuff.
A much better use for it is putting next to arty pieces or rear repair areas that are vulnerable to partisans or glider commandos.
And to everyone comparing the cost of items for OKW vs. other factions: At least other factions can build resource caches, thereby giving them an absolutely massive advantage.