Seems to me that the game would benefit a lot from having an optional level scaling feature.
First, it would allow us to have a meaningful leveling experience with our friends, even if our levels don't exactly match. (Boosting your friend is not really meaningful, at least to me)
Second, it would allow us to really appreciate the storylines present in each zone. With the way one gains experience these days, you'll only by able to complete about a third of the quests, if lucky, before having to move to another zone due to the quests and mobs being grey, even without heirlooms (Again, doing grey quests/killing grey mobs is not really meaningful). It would be a shame to let all those great storylines go to waste.
Third, it would open up new questing paths. With level scaling, one could, for example, level all the way to 85 without leaving Eastern Kingdoms or Kalimdor, since there are more than enough quests in those zones. Or one could do all the quests in Outland and skip Northrend and possibly Cataclysm content. There are many possibilities.
Fourth, it would give players more things to do at max level. Love questing but tired of dailies? Go quest in low level zones, receiving level apropriate gold and even reputation, since many of those quests award it.
I like going to low level zones and feeling super powerful.
You still can. It's an optional feature.
It wouldn't be rewarding.
Sure it would. You would receive gold as if doing max level quests. And it's a good way to earn rep with certain factions (Though you can already do that).
I don't care about this, so Blizzard shouldn't waste their time doing this.
I don't care about raiding and pvp, but you will never see me saying that Blizzard shouldn't waste their time with those things. And many people have asked for this.
What if I'm playing on a Pvp Server, downscale my level and get ganked? That sucks!
Transfer to a Pve Server. Or don't use this optional feature. Being ganked is part of playing on a Pvp Server.
Many MMO's have already implemented similar features. It is my humble opinion that it would make the game even better.