Is there any reason people always want to keep their 5 characters at the same level, once you no longer get RAF xp?

Once I take this DK to Northrend with my 4 shamans I plan on doing pretty much EVERY quest for my DK, skipping the painful collection quests on my shamans. Besides the fact that this gives me more money, lets me experience more content, and will get more achievements on my main it will also level my "tank" ahead of everyone else, making instances easier as I hit them. Also once my main hits 80 even if my clones are 78, all the rest of the quests my main does will generate tons of money.

Just curious for everyone else.