I am in a strange (though these days perhaps not that unusual) situation of speaking English as a second language but being more comfortable with it than with my native tongue. I was never in a primary school and was never officially taught any grammar rules. Yet to me, "red big coat" sounds as strange as it would to a native speaker.
A language is defined by its usage, and the rules/guidelines do shift with time. However, at any particular point there are definitely a "right" and a "wrong" way to construct a sentence. Therefore these rules (weak as they are) are definitely not just "opinion" that can be freely ignored. A rule defined by custom is still a rule.