Programs not coded for dual cores do not directly take advantage of them. That does not mean that the other processes you tend to run at the same time as your game will not take direct advantage of your dual core, however, because they do.
In that sense, even an older non-daul core game will indirectly benefit from the dual core setup, because your other applications are reaping those benefits.
In my opinion, the point is moot and largely academic. The question which is relevant is: should I get a dual core processor and will I see benefits of that when running my applications and playing games?
The answer is yes and yes; though the direct benefits for some are greater than the indirect benefits when running others. Moreover, most people purchase new equipment not simply to enjoy good performance today - but to enjoy it tomorrow as well. And it is fair to say that dual core keeps getting better while single core keeps getting relatively worse in terms of performance.
Could you get better performance for a game on a very fast single core processor than a slower dual core? Yes, in some instances this can be true. But it is increasingly less true - and is quickly becomes false when you are running other applications at the same time. Sometimes that's a voice communication package, or a bittorent client or even something like WinAmp or iTunes... it doesn't take much to observe a benefit.