Gravity bongs, thru empirical evidence to me, is harmful for afew reasons:

1) They are so easy to do and take so little weed that, generally, you will stay high and get high whenever you can. You actually end up smokin more weed in the end and you stay in a comedown all day long.

2) Eventually, again generally, that will harm your memory. It's hard to remember shit when you are high, and if you can get high so easily and cheap then you will try to stay high and not remember things from when you are high

3) The smoke is unfiltered and therefore it's not cooled.. and also you tend to take BIG hits and less of them.. That creamy smoke ravages your lungs if you get it all in there and just hold it. THat is way more smoke than most other ways to get smoke in your mouth and agani it is completely unfiltered

THat's all I can say from my own and friends' experience... I didn't notice that I was doin it until a friend turned me on to it but then I realized how detrimental grav bongs were to me.

Now I'm back to having LARGE sessions LESS of the time.. works for me