All these symbolizers have common purpose - placing labels on geometric positions:
TextSymbolizer
ShieldSymbolizer
GroupSymbolizer
MarkersSymbolizer
PointSymbolizer
CollisionSymbolizer
It makes sense to allow all possible placement options for all these symbolizers. Therefore all these symbolizers now support all these placement options:
point
interior
centroid
line
vertex
vertex-first
vertex-last
grid
alternating-grid
This unification is a result of great code sharing and simple declarative description of each symbolizer.