of course the downside is:
- older fans who stop buying because of the randomness of the event
- the generally crappy plotting and rationale of all the characters involved
- no long term plan for after the event
- and the new readers brought in because of the event drop the books when they realise the books are underplotted, frequently lose meaning in the overall history of the characters, or the characters get killed/relagated to an afterthought.
so while the boost in sales is a good short term bonus, the long term result is often a smaller customer base
(the new 52 is, for the moment,the exception to the above)