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A lot of people have become bored with their mains and are daunted by the prospect of leveling another character. This is an attempt to keep people playing by giving them a way to quickly level something they may be interested in playing in the new expansion if, for example, they are tired of healing all the time and wish they had a Warlock to play in Wrath of the Lich King.

In the end, Blizz wants money. The more people who have several accounts, or the more friends who are convinced to create new ones, the better it is for Blizz. The more people who are enticed to stay and play instead of canceling a character they are bored with, the better is for Blizz.

I think it's mostly a matter of keeping people interested. There are plenty of people who play 1-70 and still have no idea how to play well, and these 30 levels only work before 60 so you still have ~40 levels to play. I don't think it's going to change the skill level you see from people.