It happens on any case in which dirt gets inside...
I used a Speck case for about 8 months and didn't wind up with any scratches. You just have to take the case off and clean it about once a week. However, one of the clippy bits that hold the bottom part of the case on broke and made it unusable.
I can tell that you're very OCD about scratches. I'm the same. My MBP just went off for repair, they replaced the display+casing, and it had scratches on it (even though they were small) so I sent it straight back.
Don't worry about scratches with Speck.. Check out itsmemorphious on YouTube, he's used one for 18months, and he's never had a scratch, and he's like the most OCD Apple dude there is![]()