It's actually the opposite of what you are saying nikongod. This gets into mid-side processing. If you sum the left and right together you get what most think of as a mono signal, which is all of the in phase information (what our brain reconstructs as the center image). If you take the difference of the left and ride sides you get the side channel, which is obviously all the out of phase information (what our brain reconstructs as all the sounds at the peripheries). These are used in mixing/mastering. In the mixing process it allows you to do things like artificially make the stereo image wider or narrower for various tracks, while in mastering it allows you to pull out and work with things like vocals or snares which are almost always panned to the center.
As for changing the impedance, shorting the two drivers together causes them to be in parallel, which decreases the total impedance (2 / impedance of one side), and since it's an AC signal you need some resistance added or else you will get shorts between the two channels. Most headphone amplifiers now are designed to deal with very low impedances so resistors are put infront of the amp to buffer the outputs, but some are not, so crossing the channels without a buffer is considered to be a no-no. (Some reading: http://www.rane.com/note109.html )
