That depends on where we draw the line for "reduced functionality", but I think we generally agree. My point is that in normal circumstances you don't have to

That depends on where we draw the line for "reduced functionality", but I think we generally agree. My point is that in normal circumstances you don't have to drink unless you're feeling thirsty. Ignoring thirst causes some discomfort, because you're thirsty, will get headaches, cramps and so on and those symptoms will probably prompt you to drink. On the other hand, drinking too much generally doesn't lead to that much discomfort (unless you're drinking way too much), but will still be linked with various health risks.