I think they are an important part of the lifestyle. Especially important if you are just starting a relationship or playing in a multi player type scene (yes I know it is rare but nevertheless). In my experiece the partners I have had have never had to use it, but I think it gives the sub a safety net and a way of letting the Dom/Domme know that things are getting too intense.

Ultimately it comes down to choice and clear communication between couples. Communication is the lynch pin of any relationship, kink or vanilla.