I'm one who would consider themselves an up and coming "average day sailor". As such, my trips are planned to avoid uncomfortable weather to the extent possible. However, stuff happens, so some sort of foul weather gear is needed, even for those of us that will generally only be a short motor away from the dock.
Having spent many hours in non-breathable rain gear while hurtling down the interstate on two wheels at extra legal speed, I would say that the suit in question would be ok for most conditions a day sailor might encounter. The average day sailor will not be wearing their gear for days on end or taking green water over the boat. As for the aerobic level of sailing, I'd say that even crewing while racing, you don't work that hard for extended periods of time. Its normally short periods of strenous work interspersed with longer inactive periods, so I'd argue waterproof is more important than breathable for short duration wear. There is just no reason to pay for high end foulies designed for the North Sea, to keep you dry in a shower or T-storm while you make a run for the marina or anchorage that's a few minutes (or even a couple of hours) away as would be the case for the "average day sailor."
That said, the main downside of consumer or moutaineering type gear for me is the lack of specfic marine safety features as Sailingdog pointed out. Even though I had top quality moutaineering gear and equally high end gear for riding motorcycles , I opted to aquire half-decent, breathable marine foulies for my wife and I, primarly to have the safety features like high visibility colors, reflective tape and handy storage for light and sound signal devices should we find ourselves swimming.