That's what I don't get - it's the drip that poses the risk of scar rupture as it makes your uterus contract too severely (can't think of the better way to describe it).
I had the pessary only when I was induced but it was a long time ago now so it's possible that some hospitals do things differently. However at the antenatal class I went to the midwife said they prefer waters to go as late as possible because first of all the waters cushion the baby when you get a contraction so there's less likelihood of them getting distressed from contractions if the waters are still intact, and second, if they break your waters but then contractions don't start, or if it's slow, they only have around 12 hours before they get worried about infection (if they break on their own it's 24hrs but if they were broken manually there is more risk of infection).
So at my hospital anyway they don't like to break waters unless they have to or unless they are fairly sure it'll result in the baby being born soon.