This is so true. I was thinking something sort of similar to this actually as I was pondering over this. The pros to being single essentially boils down to the freedom, but too much freedom can sometimes cause a person to become lazy about their self development. Or they could become too confident in their own abilities and observations without someone there to help ground them. Having a partner to go through lifes hurdles with you whom you have a close bond of trust offers a lot of opportunities for personal growth, if you're both in a healthy place.
The criticism and pain that comes out even more once you've gotten comfortable with each other some might say is a con to a relationship, but it doesn't have to be. It gives you both an opportunity to develop the capacity for a great amount of love and that is a very very good thing. You can develop this and gain these benefits to a degree within non-romantic relationships, but I think it's even more effective in a romantic one. People for the most part seem to take heed to the words and criticisms of their romantic partners moreso than friends. I'm aware there are always exceptions to the rule, but this has been what I've observed, and it makes a lot of sense. A friend you most likely do not live with and spend most of your waking hours with. If you have spats, you can simply widen the distance between you whenever you desire without necessarily ruining your friendship. This is not really the case in a romantic scenario. When this distancing begins to happen in a romantic relationship, your relationship is in jeopardy.