I think it is very helpful to people to know you are leaving for a better fit, yes, but also that you are not leaving because you are dissatisfied with the school in general. People want to feel like they've made a good choice, it's unsettling to watch families that you respect pull their child out of what you think is a good choice school. I have been careful to talk about how great the schools that we've left have been in x,y, and z, and how we will miss p,d and q... You will get a better response if you let them know there is no "hidden" deficiency in the school you are leaving (if that makes sense)
I totally agree with this, and I was very careful to do this. Interestingly, I got far fewer negative reactions from the parents of kids at my DD's school (who may have had a better idea of the school and how DD functioned in it) than from parents whose kids do not go there. It may be that I handled it more carefully with the former group and was less on my guard with the latter--I don't know.