If the fluffy stuff wasn't cutting it for you, not to worry. There are plenty of very logical and practical benefits of summer reading. The links between summer reading and high school achievement aren't as romantic or frivolous as you might think. Completing your summer reading will directly benefit your grades, vocabulary, reading comprehension, and writing skills.
Let's start with grades. Assuming your school has given you an actual assignment for summer reading--as opposed to simply suggesting that you read--blowing it off will have a major effect on your first quarter English grade.
Let's say you were instructed to read Charles Dickens's A Tale of Two Cities over the summer. You could blow it off. However, not only will you have to deal with your parents nagging you all summer, but come September, you'll also have to deal with whatever assignment your English teacher throws at you. Unfortunately for you, your teacher will probably count the summer reading project as a "major grade" which means it's worth about the same as a test, essay, or other big project.
In my experience as a teacher, it's very hard for students to dig themselves out once they receive a zero on a major assignment because there are usually only a few more of them throughout the quarter. Okay, so let's say you blow off the reading, but then try to complete the project anyway, to save yourself from that deadly zero. Anything you earn is better than nothing, but chances are you won't be able to impress your teacher enough to earn an A or even a B on the project, since your knowledge and understanding of the novel are so shaky. So you're still starting the year without reaching your potential.
What's more, all that time and effort you're putting in to figuring out what the heck your teacher is asking about is distracting you from your other classes. It's easy to bomb an algebra test when you stay up all night desperately trying to finish your paper on "Dickens' Attitude toward the French Revolution" with relevant passages from the novel serving as your evidence. Hey, nobody said high school was easy.
Even if you weren't given a specific assignment, summer reading still affects your achievement in high school in a number of ways. A challenging novel is almost certainly going to expose you to new vocabulary, and you're much more likely to retain new vocabulary when you learn it in context, not off a random list of words. You might not even realize you have your English teacher to thank when you recognize "pernicious" on the SATs, but there's a big chance you remembered it meant "hurtful" because you read it in A Tale of Two Cities.
In a similar vein, struggling through Dickens's prose is not a bad thing. Really. It means that you'll be in good shape to read other challenging works, like those by Shakespeare and Faulkner, because you've had practice with reading comprehension. And believe it or not, reading great writers subconsciously helps you become a better writer, too. You might not notice it, but you pick up on Dickens's varied sentence structure, word choice, and sensory imagery as you peruse his novel. Don't be surprised if you start seeing beauty in your own writing after reading the masters.