I don't think it matters, my brother up and decided he wanted to write a novel and although it isn't finished what I read was AMAZING. I'd never seen him write anything before but he had a definite handle on story arc, descriptive writing and character development. I mean sometimes he would play around with character concepts but still, he never wrote any short stories that I saw and sure as hell didn't write poetry. Some people just have talent and telling someone like that that they need to slow down and start with something supposedly simpler like short stories or poetry would only be detrimental.
I suppose that could be an exception to the rule but I don't really feel that you can learn to write a novel from short stories, the two are very different. A short story has to have a conflict that can be solved within a few pages. Whereas a novel is more slow paced and should be drawn out over several chapters.
and poetry.....okay poetry does involve playing with words. but poetry is an artistic arrangement of words that aren't necessarily confined to the same standards as prose. You can do things with poetry that if you did with prose you'd just get funny looks.
Most people should have a comfort zone in which they know what they can and can't do anyway. Now I'm not saying that everybody should jump in and start writing novels but I DO feel that if they think they are capable of it why not let them try and as long as they keep working at it they will eventually succeed so what exactly is the issue? I mean deviantart is chock full of people willing to critique any writing of any experience level and there are places where you can submit you're writing and they will tell you what needs work. That's my rambling quota of the week