Sometimes is necessary some associations(many to many) in the mappedSuperClass. This associations should not be lost (inversedBy side and mappedBy ). The only one way to get this, was to implement an abstract class as Single Inheritance Table and then the mappedSuperClass inherit of this. But this is not optimum, because we are losing the OOP, creating classes which should not exist.
I realized of this when i was creating my bundle to be used for some projects, but I needed each project could extends from my class and add some properties or methods according their needs.
This is the link where you can see more details of this:
Thanks for your attention.