The water miser vent cap is kind of a neat little addition to your battery bank. The little absorbant balls inside the cap capture acid vapor created during battery charging ans return the electrolyte back to the battery. Althoug they do not capture 100% of the vapor they do significantly increase the time between battery watering intervals.
If your battery bank is at a vacation home that you don't spend a lot of time at or in a location with only seasonal acess, these caps might be a worthwhile investment. A few caps is a lot cheaper than losing a deep cycle battery. Remember, once the plates inside a battery become dry, a coating forms on them and they are rendered useless forever. Gotta' keep em' wet!
