...because the best has been Masterworked previously.
Masterwork aficionados had wondered for years how Marvel would handle the classic Avengers/Defenders War storyline that ran through both titles in the summer of 1973. Eventually, Marvel decided to reprint the entire saga in both the Defenders and the the Avengers masterworks lines. Last year it was reprinted in the Defenders line; this year it gets reprinted in the Avengers line. It seems like the storyline has been reprinted a half a dozen times over the last thirty odd years.
Make no mistake, it is a classic storyline. The clash between the two groups seems perfectly plausible, not gratuitous as in many other hero vs. hero battles. The scheme by Dormammu is reasonably clever with the only weakness being what exactly Loki brings to the table other than its ultimate failure.
But is the rest of the book worth getting if one has already read the Avengers/Defenders War storyline in the aforementioned Defenders masterwork (or elsewhere for that matter)?
If one is a completist there is really no choice. But if not, one is missing well below average material whose only long term influence was the introduction of Mantis and the reintroduction of the Swordsman into the Avengers. But that's only part of the story.
Marvel went out of their way to load this volume up with hard to find extras like Avengers features from the 1970s fan magazine FOOM (Friends of Ol' Marvel, I think). There are some nice covers, Dave Cockrum artwork, an "interview" with the Avengers butler Jarvis, a two page Forbush Man humor strip featuring the Vision and Scarlet Witch, and a very interesting Steve Englehart profile. In addition, we also get some original artwork, house ads, and collections covers. All in all, Marvel tried to sugar coat the bitter pill of buying the same storyline twice as well as it could.
Highly recommended if you passed on the previous Defenders masterwork. If you've read that then you have a tough decision to make.