As stated in the PEP, one of the primary motiviations for the proposal is to allow grammar extensions to be prototyped in Python (esp. optional static typing.) I would argue that making actual changes to CPython is much more expensive than writing a front end in Python. By adding a pgen module to Python, I feel that we are not bloating Python so much as we are exposing funtionality already built into Python.
The potential problem is that this new module must then be supported for a long time. People will propose extensions to it, which must be evaluated, and every change must be reviewed carefully for incompatibilities.
I'm not opposed to changes. However, I fail to see the value of prototyping the grammar, since you'll need subsequent changes as well, to the byte code generation, and perhaps evaluation. Also, I still doubt anybody interested in changing the grammar couldn't easily recompile Python.