Not knocking Rona. He is taller than many of our forwards, but at 102kgs he is not heavier than any of them (barring the hooker)...he's on a par with Manu. If the choice is 'buy Rona or get nobody else' I'd certainly hire him. But if leaving him alone means an upgrade in the prop position, then I'd rather that.
I don't necessarily agree with any of the old adages, rather I'm just amused at the sweeping generalisation they imply. You have to be good enough in the forwards and in the spine and have a kicking game to have a chance. You can get away with a little less genuine quality (or depth in quality) at centre and wing. If you're lucky or good enough to get a year's great value out of an up and comer, you might not have to make that sacrifice though.