It appears Nickelodeon is giving fans a reason to buy the Avatar
volumes being released next year; they're packaging an exclusive comic in each set. Here's a quote from a Video Business article about their plans:
Each of the four releases in season two will be packaged with an exclusive collectible comic book, culminating with a boxed set toward the end of the year, similar to what the company did this year with the first season.
So now fans will have to decide whether to buy the volumes and get the comics, or wait and get the season set which could have a bonus disc, but may not include the comics. "Book 2: Earth, Volume 1" comes out on January 23, 2007