I'm on Maz's side so far.
The idea that somehow, leveling through old content trains the player to perform in current content is one of those widely cited theories that may be little more than a dream. It is cited as if it is a fact, but consider:
Scrolls of Resurrection already allow a direct boost to 80.
Recruit a Friend also allows one to jump over large chunks of leveling.
Regardless of TOS/EULA/Hilda, there are people who pay to power level. They just don't pay Blizzard and feed the hackers/gold sellers.
Anyone who has taken a toon to level cap knows the drill. They know how to snooze through PUGs and BGs or get run through instances.
Remember all those SoRs? Yeah, that guy has characters that he might have been good with -- several expansions ago. Things changed.
None of those things train a player for current content at level cap. On the other hand, we have a new expansion about to hit. A boost to 80 will soon mean that you're going to start at the end of WotLK content and will have to level through Cata and MoP to hit end game. That is roughly on par with starting at level 60 today. If you don't learn the basics of not standing in the fire and such through two expansions of relatively current design, a flash tour of a few more levels of solo questing isn't going to change your performance in current instances.
If you have proof to the contrary, let's see it. Otherwise, as Maz said, nubcakes are going to be nubcakes. Apologies, no slight intended, but one of the posters bringing up the fear of unskilled nubcakes was, not that long ago, a member of that class. I mention this, not as a criticism, but as a real life example of the learning curve. This was someone who ground through to at least TBC content on a couple of characters and still hit Cata with a rather vague idea of how to even gear up for current content. Again, I stress that I'm bringing this up to show how even a fairly diligent player, one who takes the time to go to a forum and ask questions, does not necessarily gain much of an understanding of end game at earlier levels.
Did some of us learn through the school of hard knocks on our way up? Yes, but that was back in the day, when those levels took us closer to end game. I'd go so far as to assert that slogging through 80 levels of content that is no longer particularly relevant to end game may be dull enough to encourage bad habits rather than developing good ones. On the other side of the balance, consider that this could thin out some of the dead weight in lower level PUGs and BGs. It would do so directly by giving players who are just going through the motions another option and indirectly by cutting down on the number of players who are actually just a toon being jumped through the hoops by a power leveling service. Why pay for a character that is open to hacking or bans when one can just go through a regular transaction to create a new character? From the community standpoint, do we really need to train people to expect to just sponge their way through the game, half AFK and mostly carried through content? Is that really a valuable lesson? Aren't those people some of the very ones that we're tired of encountering?
While I suspect that we're going to encounter lazy, greedy, foolish or rude players no matter what options the game offers, I don't really see RAW's proposal making anything worse.