What are the advantages of a database design with single table for MtM relationships?
I don't know if this design has a name, but it is:


horrible to retrieve data with,
horrible to modify,
impossible to guarantee referential integrity.


Can't think of any advantage, but if I could, I'm sure none would trump these three.

Multiple many-to-many tables would be much better compared to this design, although I won't insist that would be the only alternative.