I have two main objection to co-washing everyday:
1) It may remove all or most of the sebum from the scalp, which is only sort of OK for very oily hair; any kind of hair needs some of it if it's going to be healthy as the oil's job is to protect our hair.
2) Unless the CO used is made at home from totally organic and pure ingredients one is rubbing some kind of chemical or another on the scalp (at the very least the preservatives), a certain amount of which will be absorbed into our bloodstream and even if they are not really toxic (which many are) our livers and kidneys still have to clear them out of the body. When seen as a single thing it doesn't seem like much but if we stop to think about the soap/body wash, lotions, creams & other cosmetics we put on our skin on a daily basis we're actually increasing the chemical load our bodies have to deal with - NOT a good thing!
So if you really appreciate your health don't overdo the cleansing, even an oily scalp can eventually "calm down" and normalize after a while.
2A/3A, medium length, layered, colored dark/medium ash blonde - "CGer" since April '02.
"Converting the 'curlskeptics' one curly head at a time..." HWHC