Where does this "more water" come from then? If the water is there, as liquid or vapour, it will form ice in the carb due to the cooling. And it will do so whether you apply partial carb heat or no carb heat at all. How can partial carb heat be worse than no carb heat (in the absence of ice particles)?
There isn't any more water. Merely that you have say +5deg ambient, in the carb it is now -25deg. At -25deg, ice will be much less likely to stick. My understanding is that ice sticks because when it impacts, the impact pressure causes the ice particle to quickly melt and then it re-freezes. If the temp in the carb -25, the impact is not sufficient for the particle to melt so it just carries on through and into the cylinder. If you use partial heat in that case, then you may raise the carb venturi to say -5 which is within the temprature range where ice forms most rapidly. Hence the recommendation for a carb temp guage if partial use of carb is required.