|laziness, impatience, and hubris|
The proposed lack of XP bonus for casting down-votes is not a reflection of down-voting being discounted as a contribution. Casting appropriate down-votes is a contribution, but you will have to be motivated to make that particular type of contribution for reasons other than gaining XP.
Casting up-votes only for XP isn't much of a contribution (but is also probably part of why the system works), and if you manage to do that for very long then I'd assume you must have an amazingly boring life. Perhaps the typical voting contribution is "casting up-votes for several reasons, only one of which is gaining XP". And that is a fine contribution. Over time, the typical human adds more weight to these other reasons and the site benefits.
And the change isn't meant to discourage people from casting the occasional down-vote. The change is meant for people who cast a large number of down-votes. They shouldn't get a large reward. No, I don't think this minor change will result in much change in behavior. The effect will probably be subtle. But I do think the large reward for the large number of down-votes is a flaw in the current system, even if just from the point of view of sanity of the system. It just feels wrong.
I often think it'd be even better to have casting a downvote have a chance of costing the voter 1 XP (for quite a few different reasons) and some have agreed with me on that point. If you cast a down-vote, you should have a good reason (as it can be a distruptive action), and should be willing to accept the same punishment that you are dealing out. That might even change the attitude of those who get down-voted. Instead of whining about how unfair the down-vote was, they might realize that 1) the down-voter got much of the same punishment and 2) someone felt strongly enough about something that they were willing to take that punishment.
I currently think losing XP for down-voting makes more sense. But I also see potential down sides to such a proposal (one big one being that it would be tough sell). So I think it makes sense to "compromise" on this point and to also take the more conservative route.
I also feel that if downvotes after a period of time are no longer counted toward XP upvotes shouldn't count either.
The proposal does make many up-votes count less over time, and I could see it going further in that direction. But I think it makes sense to allow people to be "forgiven" for their past "sins" without preventing them from being rewarded for past accomplishments. It can be quite fun to get XP, look to see what for, and discover that some old node of yours was rediscovered.