The majority of these turrets that saw action in WW2 more than justified their placement, with multiple kills and sometimes large allied formations tied up by them.
As opposed to a waste it seems uncommonly efficient of the Germans to use up these turrets in this way rather than leave 'em rusting in some yard somewhere. When as severely on the defensive as Germany became, mobility can be sacrificed for excellent placement and concealment. They're also, on the whole, a lot cheaper than custom designed Bunker armaments.
The cost effectiveness is underlined by the sheer number of turret emplacements still being built worldwide along Cold war borders postwar, from Greece to Finland, it was still considered worthwhile to recycle 'obsolete' tank turrets into useful emplacements.

Cheers,
Adam.