Some good reasons to refuse having your water broken would be:
- it removes the "cushion" around the baby which can (depending on stage of labor) a) make contractions feel a LOT stronger and more painful, and b) settle baby into an unfavorable position that might be harder for her/him to get out of without the extra room generated by the amniotic fluid;
- if baby is floating high in the pelvis, there is a risk of cord prolapse from AROM, which means an immediate emergency C-section;
- once your membranes are ruptured, you are "on the clock" to get the baby out -- usually 24 hours -- which means that everyone's anxiety about "hurrying your labor" gets suddenly much higher and the pressure to deliver quickly is more intense.
All of that being said, however, AROM isn't always bad. If it's fairly late in labor, baby is in a good position and well-engaged in the pelvis, then the risk factors are much lower. It can shorten labor by an average of 40 minutes (I don't remember where I read that, but I'm fairly certain that's the right number). And if your labor progress is slow, it's one of the least invasive interventions that they can try to speed it up. IMO, it makes sense to try AROM first, if there's concern that your labor isn't moving quickly enough (i.e., you're running out of steam and getting dehydrated, or something like that), and the next thing they would suggest is a major intervention.
My HB MW broke my water when I was at 10 cm and trying to push, but was so tired that the contractions weren't strong enough to help me get the baby out. AROM was the last-ditch effort to get the baby out at home before we transferred to the hospital.
On the other hand, if you and baby are handling labor just fine, if there's no problem with labor progress, and all is well, then there's no reason to break your water. Sometimes they just do it 'cause it's what they do, KWIM? It puts one more thing under the control of the HCP and out of the control of nature (they hate it when your water breaks unpredictably). So if there's good reason to do it and the risks are low, IMO it's not a big deal, but if there's no reason to intervene, like with most things in birth, better to leave well enough alone.