Soft release buttons are great--in the right use cases. I always carry a soft release, a full size Abrahamsson soft release, in my two Leica bags. I don't use them very often: if you keep them on the camera, yes you will get lots of accidental exposures and increase the risk they'll snag on something. I only bring them out when I need to shoot at really slow shutter speeds, say 1/15 second or slower. In that instance, and using proper technique (hooking the first joint of your index finger over the top of the release and squeezing slowly, not jabbing down with your fingertip) there is a discernible difference in the number of shots I can get without camera shake.
I don't normally use them on SLRs, because I'd think that mirror slap would negate any advantage gained by a soft release at slow shutter speeds--although there are some cameras that at least feel like they may benefit from a soft release, I haven't got any empirical data on that to judge one way or another.
The other place where soft releases are great--in this instance, Abrahamsson mini softs--is on medium format cameras. My Hasselblad 500CM, my Rolleiflex 2.8F, and my Mamiya RZ67 all have soft releases that live on those cameras. Medium format cameras--particularly those I mentioned--often have long, narrow-diameter shutter buttons. Putting a mini soft on (on many cameras, a full-size soft release hits some part of the body before triggering the shutter) makes me feel more confident, even at higher shutter speeds like 1/60 or 1/125, that I am not going to introduce some camera movement when firing it. I haven't tested this scientifically, but from experience I do believe they provide a benefit.