uxorious
Before I start, keep in mind that I have not actually been cuckolded…the following is based on my relationship with my wife, and what I have learned researching and speaking with other cucks, Bulls, and Wives. I guess my response to your question is that yes I think there is growth, although for me I wouldn't call it spiritual. To me it's more of a growth of the relationship in a number of ways. The first is the Domme / sub relationship. To me there are very few activities that can compete with cuckolding as an expression of Dominance on the Wife's part, and submission on the cuck's part. And within cuckolding there are a variety of activities that can enhance that even further. Then there is the marriage. What could be more intimate that witnessing the love of your life embrace her sexuality, shed her inhibitions, and enjoy herself completely? Those would be experiences that would be shared with others, but not in the same way as between the husband and wife. It is the embodiment of a level of trust, intimacy, and connectedness that is quite rare in many relationships.
A temptation to be resisted? I think for many that is wise advice. I don't think all relationships could handle the inherent stresses of cuckolding. But that's just something to factor into your personal risk/reward equation.
I'm not a very spiritual person in general…I take reality at face value and live life accordingly. I don't think cuckolding will lead to any spiritual revelations, or deeper truths, but I do believe it can take a relationship to a level achieved by very few. 