I'm not SBW's biggest fan, and I certainly don't buy into the ridiculous hype surrounding him, but I can't agree at all with this particular analysis. There is no doubt that SBW deserved a spot in the AB's RWC squad, as he is clearly the second best 12 in the country. If you disagree with this, who would you have taken in his place? SBW was the form 12 in the entire competition over the first half of the tournament, but faded a bit over the later half of the competition (due to injuries / perhaps concentrating a bit too much on boxing), but was still playing at a pretty high level. Nonu was rightly given the AB's starting jersey during the key AB's games, but SBW was clearly the second best option if Nonu had been injured. I personally wasn't a fan of having SBW on the bench during the key RWC games (I would have preferred Toeava's versatility), and found the decision to start him on the wing against Argentina in the semi-final slightly bizarre, but I still think he clearly warranted a place in the squad.
I'm not sure why you think his defense has been awful - indeed it has generally been one of his strong points! His work-rate on defense has been top notch, and when he's been in position to make the tackle he's almost invariably made it. He has been caught out of position at times on defense, but this can happen to most players (especially players new to the game) - indeed Dusautoirs try in the final was due to Nonu being caught out of position in defense (by not coming up in the line fast enough). I would rate his defensive ability at least as good - if not even slightly better - than Nonu's. My biggest criticism of SBW as a rugby player is the fact he seldom hits the line at full pace - he often just jogs up to the line. This means the defense doesn't have to fully commit to him, hence they can cover the man that he is offloading too. On the odd occasion that he hits the line at pace he can be incredibly effective. The defense has to fully commit to him - if they don't he goes straight through, if they do he can offload to a player in a gap. If he could stick with rugby I think he has the potential to be a truly world class player, but I don't think we will ever get the chance to see this...