Because what you've written doesn't make much sense? You can't pass a type as a function parameter to a user defined function, and if you mean the builtin "new" (no capital) then that uses the normal allocator.
I think he meant that MyType is an enum and New is a user-defined function with a huge switch that does allocate (with placement new?) the correct type based on the value of MyType. Yea works, but introduces runtime overhead and is the gate to maintenance hell.
