Pay-for-use reduces the use from the poor people, because in relation to their economy it is a huge cost. For the rich people, pay-per-use has no effect what so ever, because for them the tiny amount of money they'd have to pay doesn't have any effect on their economy. In fact, pay-per-use can even have the opposite effect, as that would make water a status symbol. Similar to when rich brats go to the restaurant, ordering the most expensive champagne and then telling the waiter to pour it out in the sink - only to show off that the money they just spent on the most expensive champagne doesn't mean anything to them because they're so rich. But perhaps that's just a Swedish phenomenon (it's even got a word, "vaska" in Swedish).
It would help to know what you mean when you say "pay for use." Like going to the liquor store and buying a water bottle?
Do rich people become rich by wasting their money on resources? If anything, pay-for-use reduces use by the wealthier - unless they see some benefit from paying more (like toll roads, which are often quieter and quicker than non-toll routes) - because they will end up paying the most.
It's unfair to "ask" someone to pay for something they don't use, regardless of their income. Tax-funded utilities "ask" everyone to pay for a resource regardless of their use of it - fundamentally the least fair method of all.
The problem is that water isn't like, I don't know, your cell phone minutes. Everyone uses water. Everyone. Hence, the discussion of water being a public vs. private good.
You may be right in stating that these companies have managed to bring the price down of water... maybe that's true. The discussion here, though, (and noting all the distinctions you provided in your first post) is whether a company like Nestle can claim, rightly, that the water it finds at a lake or running down a river, is theirs if they treat it with whatever water treatment system they have. That's the whole gist of privatization of water: selling water as if it's your own.
What Nestle should be saying, I think, is that treatment of water should be privatized, but not the water itself. And that should be the case because water is unlike your computer or your PS3 (i.e. if you get the parts of a ps3, build it, and sell it, then sure, it's YOUR product). But with water, not really buying into that argument that it belongs to any one person. It belongs to the public. Thus, no one person can claim a right to it.
This reminds me of a story from Israel: A school was having a hard time with parents coming late to pick up their children. They tried appealing to the parents sense of justice and even shaming them but nothing worked. They decided to charge the parents money on a scale according to how late they were. The result? The number of late arriving parents increased!
Freed from shame or guilt, they treated the school staffs waiting time as a mere commodity, and thought no more about it. Because they could afford not to care.
The number of late arriving parents increased? So it just drove people out of schools (the number one reason I'm against privatization of schools, btw). This comes from someone whose mother was unable to afford continuing to school because her parents were too broke to pay for things like books, school supplies, etc.