Most shampoo companies (that are not professional product) use it becasue it is a cheap cleanser. However it is kinda harsh for most poeples uses. It will strip artifical colour out of your hair as well. So its best not to use a product with it in it if you can avoid it.
I have unfortunately inherited my grandmother's super-fine hair, and now that I am in my mid 30's it is starting to thin out. I started using shampoo bars that I buy from Etsy, from someone who lists all her ingredients, so I know exactly what is in them.
I expected the improved general hair condition, no longer having to use a seperate hair conditioner as my hair is no longer being stripped, etc. A huge added bonus side effect has been that the rate of thinning has stopped, and might have reversed a little. ( I am fully prepared to admit to myself it may not have reversed, just that it has more volume from not being weighed down, but there is *way* less hair in my comb) I am convinced it is the lack of SLS in the product.
And just so I am not straying too far off topic here, my favorite useless product ever was the flowbee. You put an attachment on the end of yur vacuum hose, and started vacuuming around your head, and the attachment would cut your hair as it was being sucked up.http://www.youtube.com/watch?v=V3G1cwqYkO4
"just bounce it all over your head!"