I am not an engineer but I will defer to any expert opinion on this subject.
That said, I've had one of the cheap grips for almost 2 years and have never used my K5 without it attached. I shoot lots of seascapes and high fog/moist areas and my camera very often gets wet, quite wet.
Occasionally after a drenching, I will detach the grip to check for damage and I never find much more than a little wetness just inside but never near the contacts. The assembly screws have gotten rusty where exposed to seawater, but I don't get too worried about that. Honestly, I still can't believe I got this grip for $45 bucks or whatever it was. It has been worth it!