Any writer who's been around me any length of time at all knows that I don't want to hear "they say" in a critique of my writing. I've been doing this long enough that it makes we want to scream when someone pulls out some rule that often doesn't even apply because they don't know what the rule is meant to accomplish.
The "rules" will teach you to write well but to really get good, you need to understand the reason behind each rule. If you don't, then you'll never know when to break the rule. FREX, you'll be told not to used -ing verbs (progressive tense verbs). That's because too many beginning writers don't understand that this verb form implies continuing action. Used in a complex sentence, it can also imply simultaneous action. This creates problems when you see a sentence such as:
"Opening the door, she crossed the room."
Uhm. I think not. Not unless she has extremely loooong arms. (This rule, along with a number of others, is enumerated and explained in the Turkey City Lexicon which can be found at: http://www.sfwa.org/2009/06/turkey-city-lexicon-a-primer-for-sf-workshops/)
There's also a "rule" about gerunds. (Gerund is a progressive tense verb used as a noun. e.g. Running is good exercise.) The "rule" says don't use gerunds (though I suspect this often comes from new writers who can't tell a noun from a verb and so are applying the previous rule wholesale.)
The real "rule" is that gerunds slow the pacing. Well, gee. Maybe I want to slow the pacing. Maybe this is a good place for a gerund. See what I mean? How would you know unless you understand why that particular rule exists?
I suspect that the codification of the rules happens because new writers tend to make the same mistakes over and over until more experienced writers are tired of explaining over and over why they need to change what they're doing. They become the writer's version of "because I said so." That makes them dangerous because new writers pick them up and share them among themselves, spreading their non-wisdom like the bubonic plague.
So my advice to all new writers is: whenever someone tells you "don't do that" but they don't explain why, ask. If the answer comes back as some version of "they say", stop and think about it. Does the advice make sense? Do published authors whose style you admire do this? If they do, is it only rarely?
Few writing rules are truly unbreakable, but you need to understand what they mean to accomplish before you'll know when to break them.
When it comes right down to it, the rules have a lot in common with the Pirate's Code; they're more like guidelines. You were never meant to follow them off a cliff.