Exceptions to the "rule" could seem arbitrary. For example, we say "more fun", and it is incorrect to say "funner". So what's to stop someone from feeling more comfortable saying "more red" than saying "redder"?
There are times when rules are better called "guidelines" than "rules". So while it's correct to say "redder", I would not necessarily say that a native speaker of English who says or writes "more red" has poor language skills or that breaking this "rule" is representative of uneducated language, necessarily.
Some of the more intelligent people in the world often speak on National Public Radio, and I hear that some of these speakers sometimes don't follow all the "rules" as codified in English grammar books and references and as taught, perhaps in a very rigid way, to English language learners.