Why is joint ownership of a patent usually a bad idea?

Joint ownership is usually a bad idea for several reasons.

First, if the patent is infringed, all joint owners of the patent have to participate in the patent litigation before that suit can move forward.  If even one of the joint owners does not participate, the other joint owners cannot move forward with the case.

Second, unlike joint owners of a copyright -- joint owners of a patent are free to license their interest in the patent without accounting to the other joint owners, and without sharing any of their licensing revenues.  This usually results in a "race to the bottom," where some joint owners try to undercut other joint owners, getting whatever value they can from the patent as early as they possibly can.