What I've read says that self settling is related to self soothing, which in turn is related to resilience. ie not needing someone (or something) to 'soothe' you every time you fall over, have a headache, can't sleep, things don't go the way you want them to etc All the skills that we want to have as adults. I'm not sure exactly how that is relevant to a small child, but that is what I've read.

Having said that, as a parent, one of my 'jobs' is to raise someone who can function in the world, on their own when they need to. Just like we teach our kids to eat with a spoon or fork, even though we could sit with them at every meal and spoon or put the food into their mouths for them, it's because we want them to be able to do for themselves. Having said that, it's a developmental thing and we don't persist with a spoon/fork, withholding all other means of eating from our children until they can successfully eat with a spoon and fork for themselves, do we? They do it when they are ready. Same with self settling. DS is an awesome self settler, which is perfect for me. I couldn't handle co sleeping, laying with him til he fell asleep or cuddling him to sleep (I'd feel like I was being 'controlled' and end up very resentful) but I also couldn't handle him CIO or getting frustrated and upset trying to do something he clearly wasn't ready for.
I think it's up to the individual. For me, self settling is important. When my LO is ready. But if I felt like Cherished (for example) self settling would not be important at all. I love that she feels so passionately about it - it makes her an awesome Mummy