We need to break it down more. Braids with extensions (especially synthetic hair) are NOT good for your hair. It may seem to help your hair grow but it's not; nothing can make your hair grow really. The reason it seems that way is because you aren't constantly styling your hair so you have less shedding on a daily basis. Some of the hair that comes out when you take your braids out is just the natural shedding that was not apparant while the hair was braided. Some of it is breakage from build up from leaving braids in too long. I have too many braid horror stories to count (that would have to be its own post/topic).
Just like anything we do too much, it can cause problems. If we stress one part of our hair over and over again (i.e. wearing the exact same style all the time), it can cause problems over time. Corn rows done too tight or too often can cause alopecia (hair loss). I developed this problem after years of having natural hair thinking no perm = no problems. Not so fast. I always wore the front of my hair corn rowed because I was never a fan of the mini fro, so my hair was always stressed in the front and now I have some places in the front where my hair broke off and is very slow to grow back. Mind you at the time I didn't think my hair was even braided tight.
I think variety in hair styles helps. Braids aside, if you wear a pony tail everyday you will notice over time the back of your hair may start to break off because you are constantly pulling it into the pony tail (regardless of your hair length).