Ideally, SparkNotes are meant as a study aid and supplement for literature. In reality, most students use it as a replacement for literature.
I was in high school 10 years ago and easily the smartest guy in the class, and even I occasionally had to take the Cliff Notes shortcut to finish a book report on time. If the guy with the highest grades was forced to skip the reading once in a while, how likely is it that the mediocre students will use book notes? In my lit classes, only a handful of the students didn’t use Cliff Notes.
Thinking back to college, I never used book notes of any kind. It wasn’t conscious, I think I just had a handle on my assignments. But in any case, I can’t think of anyone using Cliff Notes, SparkNotes, MonkeyNotes or any of the others. I think that at an undergraduate level, book notes just aren’t enough to get through a class and/or assignments. You have to actually do the reading.
So, do book notes ruin literature? Only on a low level, to those younger students who can actually get by using them. I pity them, because they’re missing out on something wonderful. Once you get on to serious literature, book notes are simply not capable of replacing all the things that make literature great.
On a side note: I’m working my way through The Fountainhead right now, and I imagine I’ll browse through a book note once I’m done. Having finished college, I don’t discuss literature in a group setting anymore, so it could be useful to see if I missed anything.