I believe the main benefit of "zero deck" is to help achieve effective quench meaning about .040" clearnace between a good sized area of the piston and head - this with a closed chamber head. A similar result could be achieved with a FT piston below deck and a thin head gasket or with a piston below deck with quench dome.

Its just easiest - $ and hours of labor - to do well with a zero deck piston, closed chamber head and .040 head gasket.