I love cowashing and will forever cowash. Cowashing freshens up my hair after exercising. I exercise at least four days a week and a cowash follows it. Also after I cowash I use a creamy leave in (Shea Moisture). I still use shampoo and clarify when needed. When I get deep into my stretch, I shampoo every other week but I condition and steam weekly. We don't have hard water so I think that helps my hair.
I haven't experienced any weakening of the strands, as a matter of fact I have experienced the opposite. My hair is stronger, thicker, and longer. I love it.