ICv2 via The Beat:

The most disappointing aspect of this summer's comic "events" was the performance of these "tie-in" series, and when retailers talk about "event fatigue" in the comic market, it's these ancillary books that they are talking about. The action in these "side series" mirrors what is happening in the primary event book, but it usually doesn't matter much in the outcome, and fans are increasingly resistant to these miniseries that try to ride the coattails of the main event book.

Of course, a good creative team can make one of these "side series" better than the main event.