Isn't choosing the semantic of a word based on the context?
Anyway: The bad thing in rules and guidelines is that if you put limits or discourage something, people will try to use the borderline case every time.
Deleting it seems a better choose, you won't discourage neither encourage it and people will use it when they feel it's good because they don't know it that much and they will choose that shapes only if it they think play good. If you instead make people know when they should use it, they will use it, saying it's not a rule and the other rule don't cover this as it's a separated guideline, even if it doesn't have sense.
Anyway: The bad thing in rules and guidelines is that if you put limits or discourage something, people will try to use the borderline case every time.
Deleting it seems a better choose, you won't discourage neither encourage it and people will use it when they feel it's good because they don't know it that much and they will choose that shapes only if it they think play good. If you instead make people know when they should use it, they will use it, saying it's not a rule and the other rule don't cover this as it's a separated guideline, even if it doesn't have sense.