I know World of Warcraft just released patch 2.3 which increased the amount of experience you get for levels 20-60, which has made a lot of people happy, not just for their alts, but because now when their friends finally stop fighting the urge and jump into Azeroth, it's not to hard for them to catch up (at least in level) and play with their friends. I think these sorts of changes are really interesting, because it shows games desire longevity, and return customers. But it also makes me wonder why they don't just do it in the first place?
I never really understood the idea of the level grind being such an arduous task, when there is so much game content out there that players could better spend their time experiencing. When I first left FFXI for World of Warcraft, I was so excited by the prospect of fast leveling (or at least, a lot faster than I was used to), the prospect of getting to end game content in 10-15 days /played was fantastic. Because when I hit 75 in FFXI I was at 111 days /played, which is absolutely ludicrous!
I think that MMO designers should start expanding on optional content, story lines, lore, end game raids and instances, instead of the tedious grind for reputation, levels, pvp rewards. Allow PvP to be more focused like the WoW Arena system, make end game raids easier for people to access and more repeat friendly. And most of all, make storyline more of a pressing matter than just repeatable text and cut scenes that people skip over and ignore.
A lot of amazing content is being lost or ignored just because of burn out from endless level grinding. By the time people get to end game, they are often sick of parties and grinding that all they want to do is just get on with something more interesting and fun than more grinding. So by removing the tedium from the level grind you are removing the burnout that people experience after levels and levels of constantly fighting the same monsters and pressing the same buttons and building the same parties for the optimum efficiency.
Another thing I'm seeing FFXI do, and I'm sure other games are as well, is using in-game incentives to bring people back to Vana'diel, and this one only makes the level grind even quicker, with the "Anniversary Ring" which increases experience gained for up to 720 minutes of play time. Twelve hours isn't a terribly long time in terms of leveling in any MMO, but if you can make the first 10 or 15 levels go by even faster, then I say that's a pretty good incentive.
All in all I think most of it is just a way to get subscribers back to the game, instead of actually wanting to make the leveling process any faster, but I think the benefit is the same for new players, returning players, and people who already play the game and are leveling new characters.
All I know is, that it made the decision easier to pick up an old game again, and see some friends I haven't talked to in a few years. And that's a greater benefit to me.