I see this a lot. It may be true that rulers are more accurate, but I've always struggled to read the small increments accurately. I've come to the conclusion that perhaps it's a form of dyslexia; I'm fine with writing, but I find numbers and rulers difficult, particularly numbers in small point sizes, which appear to jump around. A chain gauge, on the other hand, is dead straightforward. I think the trick is to know exactly what it read when the chain was new. Now that I'm using the same chain on both my road bike and mountain bike, that's quite easy, and I can clearly see any deviation from that.