Forums and lists that don't publicly enforce generally accepted rules of net etiquette tend to lose their more knowledgeable members. I, and I include myself in that class, have witnessed such collapses; they're not pretty. An example may be found at DigitalPoint's html/css forums. Except for a very few of us who can't resist the challenge of providing guidance, there are no authoritative voices left.
A common apologia for cross posting is, "I didn't know which forum was best for my problem". My answer is, why not? It is proper netiquette to lurk in a forum for long enough, say, a month if it's busy, to learn forum rules and just where their expertise lies.
The important argument against cross posting lies in not pissing off those who could help you. There are few things in forums more irritating than taking the time to unsnarl someone's markup and css, figure out the fix, and post it only to find you've wasted your time because a duplicate post in another forum has already been answered. Even if your helper never found out about the other forum, you've still wasted his time. If you insist on cross-posting, it is your responsibility to say you've done so, and link to the other threads.
You may disagree with my stand, and that's alright, as long as you accept that you will be publicly chastised for failure to follow the rules.
N.B.: See How to ask ….