Putting a character in a kids cartoon book/tv show doesn't automatically make them a lame, basic character for young kids. Darth Vader isn't a terrible two dimensional cartoon character, and neither is the Emperor or Tarkin or Ahsoka or Thrawn. People can take good characters and do them badly, but it doesn't make the original character bad. The Rebels cast were terrible cardboard cut outs from their very inception, nothing can fix that.
Why would anything with the cartoon characters is anything other then a kids book? The character's are too two dimensional to be in anything else, and you don't make a normal novel with cartoon characters on the cover when the characters are from a cartoon for 5 year olds. Its like writing a book about Barney being a psychotic killer, its ridiculous. When a character is specifically designed for a young demographic, you don't start putting them in books for older people. Not that any writer on Earth could make Hera or Kanan (or any of the other Rebels cast) into actual good characters, but its not something that makes any sense to attempt in the first place.
Now, maybe they tried to stick those bland, generic saturday morning cartoon characters into a book with a story too mature for the normal 5 year old demographic, but if they did that makes New Dawn possibly the biggest screw up in Star Wars book history. At that point it might be worth checking out just to see the car wreck of a "novel" that tries to make two kids tv cliches fit into a normal SW story. Not that I will (I'm not a masochist) but that would give it a reason to be read, I guess.
In your opinion its not, which is fine. My opinion differs, obviously.
I know why a consumer would buy them, I was talking about why the collection was made in the first place. I bought it because, used, it was only about $1-2 more then the Tarkin paperback, and I was willing to spend that difference to get the short stories. The cartoon "novel" is useless filler (although it might have a use if I ever run out of toilet paper

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Because being a professional book company makes them infallible? As much as I love the books of the old EU, its not like it didn't have some huge mistakes when it came to real world stuff (like basically forcing the best SW writer, Karen Traviss, away from SW books forever). The SW books are, in the end, run by people. People aren't infallbile, and its not like they have top quality people running the new canon. I have faith in individual writers, but not the people running the new canon (especially since Dave Filoni is pretty much part of that group).