Blended Relationship: Monogamy with Polyamorous Partners

It can work just fine so long as everything is open and the choice to remain monogamous is left up to the individuals in the relationship. My relationship has been a mix of poly, swinging and mono and it works because it's the choice of each member what, where and with whom they have a relationship.
In relationships where there is an element of kink such as master/slave the roles should be chosen not forced. Even in cuckolding relationships the dynamic is best when the parties involved have chosen their roles.
This should also include all aspects of the relationship, in a poly relationship the emphasis isn't just on sex so the monogamous partner should understand that his/her poly partner is going to invest time and energy into the other relationship(s). The mono partner needs to be strong, self sufficient and able to fill their own time and needs.

In my relationship my mono partner is free to choose to have another relationship if he wants one and I spend quite a significant portion of my day with him...but there are times I need space to nurture my relationship with my other partner. Since my partners are good friends it works because they each want the other to be happy.