My take on this is that learning the rules is more important for social reasons than aesthetic ones. Many of these rules can guide you towards aesthetically pleasing choices, but as far as purely aesthetic goals go, it's more important simply that you like the way that you look than that you have adhered to the many rules of dress. On this count alone, it's much better to like the way you look while breaking some rules than the other way around.
But the "rules" tell you what's appropriate for various social settings, what will make you blend in, and what will make you stand out, what will make people say, "nice [insert odd accoutrement here]", what will make people you to refer to you as, "the guy with the [insert conspicuous rule-breaking item here]".
In some settings, depending on who you are, this may be fine. In some settings, it may be downright rude. Knowing the "rules" of dress in menswear is no different from knowing good manners. Deviating from proper decorum depending on your position in different social settings is just as much a part of good taste as knowing what are appropriate deviations from standard rules of dress. For example, if you're the guy wearing the tan suit in this picture:
Originally Posted by Caustic Man
then you damn well better be the dude sitting at the desk rather than one of the dudes standing behind it.