The summer film slate, which used to be bookended within the actual summer season (when kids were out of school), has become bloated and over-stuffed. Beginning in late spring and tapering off throughout August, at least one major release punctuates every single box office week - with many weeks now seeing multiple films battling for box office revenue. As a result, plenty of quality (along with a fair share of not-so-great) films are falling through the cracks, and there's less incentive for viewers to return for repeat viewings.
Opening weekend has always been a strong indicator of which films are going to turn a profit and which ones are going to have trouble earning back their budgets. However, in a lot of cases, a major factor in box office success is a film's staying power - fueled by casual moviegoers responding to word of mouth along with second (and third) viewings by established fans. In order to secure over a $1 billion total haul ($500 million domestically), The Dark Knight was number 1 at the box office for four consecutive weeks back in 2008. Iron Man 3 only managed to hold the number 1 crown for two weeks, before another major tentpole film, Star Trek Into Darkness, took the top spot. Fun fact: the following week, Star Trek Into Darkness came in at number 3, bested by both Fast & Furious 6 and The Hangover, Part 3 (Iron Man 3 was number 5 at that point - four weeks after release).
As a result, studios would benefit from spacing out releases - or outright bumping major films that aren't summer season worthy into the fall, winter, or even spring. As it is, the release slate is cannibalizing - stifling potential longterm successes and outright killing-off films that are more difficult to market.