I did not suggest downvoting purely on account of disagreement. It is true that if I had not responded to Gondolinian's comment, I might have downvoted it. But not just because I disagree, but because I find complaints about downvoting unpleasant and would rather see less of them on the site.

In general there might be many other reasons for downvoting which do not necessarily involve disagreement, such as vagueness, excessive verbosity, illogical reasoning, and so on. Again, of course you can simply respond and mention those things, but again in that case there is not all that much reason for downvoting at all. The advantage of downvoting is that it takes very little resources and does not require responding to something which may not be worthy of a response.

The suggested policy does not necessarily fail, for several reasons: 1) the person may indeed in some cases realize why he is being downvoted; 2) even if he does not, he may speculate randomly and modify his behavior until he is no longer downvoted -- i.e. downvoting provides selective pressure on comments; 3) in some extreme cases, it would be good even if he just becomes less likely to comment at all. In any case, as I said, the point of downvoting is that such a small use of resources is involved that it is not necessary that there be some particular positive effect in every case.