I have never gotten a blowout, so i am speaking purely in theory. or in my humble opinion. I think blowouts are a good thing to get done ON OCCASION. It keeps the cuticles smoothed and the hair trained, and when the cuticles are smoothed it feels like the hair isn't thirsting for a moisturizer at the end of every day. The results are always gorgeous. But if you get it done every week or every 2 weeks, you will start to notice your hair thinning out. I have a friend that gets hers done often...she has thick hair and her hair is soooo swingy and gorgeous, but i notice her hair thinning out, and then they cut 2 inches of it, to combat the thinning. I really hate to see women with swingy hair that is obvious that they've had a cute blowout, but their hair is like shoulderblade length but is super duper thin. I really hate it.
I believe the thinning is a result of the stylist stretching your hairstrands with the brush and holding your hair taught. Some of the hairs are stretched too much, and are stretched and stretched and stretched every time you visit the salon and then finally the strand can't take the stretching and it breaks. this is what is my opinion on how too many blowouts can be a bad thing.