It's true that multi-threaded code is meant to work on more than one core (be it two or more cores available) but it's not easy to multi-thread all kinds of code. Something very linear can be done easily (things such as encoding where it's the same instruction repeated over & over on a huge amount of data) but games are not so linear.
ID Software have had SMP code since Quake 3 & it's never worked properly. IMO, if someone like John Carmack is having a tough time getting it to work, it doesn't bode well for the rest of the industry.