Do crossovers help or hurt comic storylines?

Marvel’s Civil War story arc was huge; it gave the publisher a huge boost in popularity while also giving new readers a great starting point to pick up on the characters involved. The entire Marvel Universe was effected by the events of the Civil War, so it was very interesting to branch off and explore their stories afterward. About a year later, Marvel coordinated events in their Universe to have another crossover event; called the Secret Invasion – which involves the infamous shapeshifting Skrulls, as they attack Earth in an attempt to take it over; while many alleged heroes are being discovered as under-cover Skrulls themselves. Another event that involves the entire Marvel Universe, and effects the way the individual storylines run thereafter.

The trend is no secret; Marvel has been promoting their crossover events as a way to get people interested in their comics and brands. Who doesn’t want to see Iron Man and Spider-Man on the same few panels of a comic, or even duke it out a few times? It’s simply exciting comic reading; just as TV shows create a buzz in their storylines for ratings and viewership. However, what does this do for the characters themselves? Have crossovers turned the plots in between into fluff?