I don't really see how you'd get a positive ROI if you factor in the previous assertion "the price would plummet"

As I wrote above, I'm happy and eager to learn what I don't know about BTC, and I love the spirit of it. Unfortunately, according to the calculations on
http://www.quora.com/Is-Bitcoin-doomed-to-fail it looks like the ROI of such attack would be positive.
After such an attack:
-The attacker is happy, because he now owns good and services with intrinsic value.
-The BTCs owners could be very sad, because panic could trigger a drop in the FRN/BTC exchange ratio, triggering evaporation of purchasing power of their BTCs, e.g. their BTCs can buy much less goods and services than before the attack.