There's actually a huge volume of trades during the dips, at least according to bitcoinity.

I still don't understand why they would bother spreading their attack volume across multiple sites. Hitting Mt.Gox with everything they've got offers two potential strategies:

- trigger a crash, buy low, stop the attack and watch it go back up

- trigger a price drop and profit massively from arbitrage with other exchanges still trading higher(BTC China for example)