Thanks Matt!
That is certainly a valid argument regarding implicit many-many relations, but not for the reason you have described. As I wrote in 339, I think implicit many-many is a solved problem: Introspection and Lift should both simply follow our convention when printing and reading the schema.
The main reason I am interested in including the implicit many-many syntax is that they enable a simpler Photon API for the majority that does not need extra fields on the relation table.