In the 20-plus years that I was involved in poly relationships, the one factor that came through loud and clear, despite all the good aspects (otherloves spending time to share interests that the primaries don't share, etc.), is that once A and B (one or the other) agree to add C to the equation, there is (obviously) less time for the primaries, and as a result of the NRE of B and C less in all other aspects of relationship that was previously held as "special" to the primary bond. The end result of which is your wife or husband primary (in my example) may feel reduced to "just another partner"...and the sense of "special-ness" that you had when you agreed to be life-partners is simply not there anymore.

Speaking from personal experience, having been A, the NRE of B and C, can cause both compersion and jealousy at the same time.