I am a 60s throwback, earth-lovin, tree-huggin', natural livin', hippie mama ... so I pretty much only knit with natural fibers. :XX: Yes, even for my kids. And I prefer to use yarns that are as close to their natural state (untreated, minimally processed, organic, etc.) as possible, though I don't always maintain that standard. So wool, cotton, alpaca, cashmere, mohair, bamboo (absolutely
it!), etc. make up the majority of my stash. (I do have some blends, especially sock yarns with a nylon content.)
I try to dress myself and my family in natural fibers, use natural fibers in my home, avoid chemicals in our home and body care items, eat traditional foods (i.e. whole foods, not chemicals or anything engineered or hydrolized or hydrogenated, organically grown/grass fed as much as possible) and generally live a simple, natural, low-impact kind of life. So, for me, natural fibers fit into my idea of healthy living (healthy for me and my family, and healthy for the planet).
Also, I think MrTea brings up a valid point. If I am going to invest my time in a project, I want that project to be worth taking that time away from other things in my life. Does that make sense? For me (just personally speaking here, so just FOR ME), synthetic yarns don't fall into the "worth it" category. They don't make me happy, I don't like the idea of using them, don't like the actuality of working with them, don't like the idea of someone I care about wearing/using them. (Your skin is your largest organ, remember? And you can absorb all kind of stuff via your skin, so wouldn't it be better to wear natural fibers, sleep under natural fibers, than it would be to wear something petroleum based, made out of plastic, yucky?!? I think about these things when I'm knitting.
As Alison mentioned, this is just MY take on things, and I don't feel that it is my place to impose that on anyone else. Doesn't mean I won't suggest alternatives, but I'm pretty much live and let live when it comes to yarn (and a lot of other things, too