AFAIK XNA 3.1 had PrimitiveType.Point but XNA 4 didn't. Same for point sprites (particles) IIRC point sprites were no longer supported in DX10+. I don't know why they removed PrimitiveType.Point though.
MG tries to be XNA 4 compliant, that's probably why there isn't a PrimitiveType.Point : it's not even in the XNA 4 documentation.
EDIT: BTW, the XNA 3.1 -> 4 cheatsheet ( http://www.nelsonhurst.com/xna-cheatsheet/ ) says to use PrimitiveType.LineList instead of PointList. It is possible that using LineList duplicating the points is enough to render points without modifying MG. I haven't tested it though.