So...

I'm not a member of any of our dev teams, Evenios, so I am not going to be able to answer that question with their reasoning. I can, however, give you what I feel is the most likely answer.

If I have an account setup, and have an established character that I like to raid or PvP with, I am most likely not going to want to drop everything I have going on to level a new character with someone. There may be times when I want to play my main character, and my friend is going to want to work on their own. Well, with them being able to grant me levels, we can feel free to take some time away from playing with each other and I don't have to worry about catching up to them while they are not online-nor will I have to worry about missing out on the triple experience, due to being too far behind them.

Like I said, this is what I believe to be the reason, not a definitive answer, though. :0)