dual core is a very nice thing to have as it makes your computer feel VERY fluid. at almost any given time there is always a cpu ready to take instruction instead of having to wait for something to finish first. ie, you want to open firefox, word, winamp, and update your virus scanner as fast as you can click them... have you ever gone to click on your start button and had to wait a 1/2 to 1 sec (God forbid) for the menus to pop up. dual core/dual processor largely keeps that from happening.
as to most programs not being able to use dual processor that is true, except, any current operating system can send individual threads to any available processor meaning that 2 programs running concurrently can each have their own processor even if they think that processor is the only one in the system. multitasking is the dual processor's power.
as previously stated, for such a small increase in price (when the price drops occur), there is no reason to not have dual processors. if you run more than 2 or 3 programs at once (including virus scan, web browsing, mp3 player, bittorrent, etc) then you will get far more enjoyment out of dual processor.
when you start banging on photoshop while downloading the latest release of the debian distro (many many discs) while watching a divx video to pass the time for photoshop to finish the render, you will greatly appreciate the dual processor setup. will you take advantage of dual processors all the time? perhaps not. but is <$50 worth it for the times when you do? that is up to you.