First of all, thanks for the post. I always want to discuss monogamy on this forum as well and there have been some great conversation starters here.
I think that every relationship style comes with it's own pluses and minuses.
I think that monogamy gives two people an environment in which they learn to explore each other and themselves more deeply.
In an open or poly context, if your partner in not into a certain sexual position, for example, you can just find it elsewhere, never having explored that with your partner, never having talked through what it meant for them or for you, never deepening a part of your bond that could have been deepened. For lack of a better work, it can "force" you to dig in order to keep a relationship fresh and alive.
The beauty to me is that you commit to being in a position that will both promote growth within each other while staying with each other and only each other for a very long time.
Of course, it is possible to do this if not monogamous, and it may not happen if you were monogamous... ah caveats. Again, there are very similar pluses and minuses to each... but they can both be honest and beautiful.