"It's no different than us walking up to a car and saying 'Why isn't that a hybrid?"
Well, the answer to that is that Hybrids are more expensive, not as green as their pious drivers think (not by a mile), not as fuel-efficient as their drivers think unless they're just being used for really short trips and are mains-recharged (making them effectively a poorly designed range-extended _electric_ car), and are just used for eco-posing by tossers in exactly the same way that the Guards Red 911 was used for posing in the 80s.
By the sounds of it, if programmers can start to take advantage of massively parallel computing then it'll be a hugely better idea than Hybrids. Games shouldn't be too hard to code once the basic ideas behind them are sorted out (you can break down the tasks into concurrent threads pretty easily), and I'd imagine that massive parallelism would be perfect for database applications- it's a lot of simple functions run very quickly. Bang, there's two major segments already on their way.
You then have web servers- they'd be good on a parallel system for the same reason databases would be.
Chips like the Parallax Propellor have even been getting hobbyists used to the idea.
So anyway, it looks- to me- like there aren't really any huge drawbacks to running massively parallel architectures except they're a bitch to program for as you can't just flowchart a program- you need multiple flowcharts running simultaneously and still interacting. Crack the programming and it'll quickly take off; don't crack the programming and it'll fall by the wayside.