I don't want to stick my head in a bees nest with my second post, but IJ Reilly is absolutely right.
EVERY operating system, including OS X, will experience memory leaks at some point. This occurs when software does not use memory anymore, but for some reason does not inform the OS the memory is released. This memory will remain allocated while it is not being used by anything anymore. Mind you, the OS will ONLY free allocated memory if the program using it will tell the OS it does no longer need it. If the program does not do this due to a bug or whatever, the memory will not be released, not even if they program originally using it is closed and no longer running.
In time, there will be so much memory allocated without being actually used, swap space is needed to keep the computer running. This memory does not even have to show up as used, as it can just be reserved. As a result, the computer will slow down and the only way to return to full performance is to reboot. An increasing swap space is a clear and dead giveaway for a memory leak.
These memory leaks typically do not occur too dramatically in the OS itself, and when they do they are usually quickly fixed by an update. However, some software does create these memory leaks and they sometimes do not get fixed quick or even at all.
This quickly explains why some people are having problems with this and other do not: not everybody uses the same software or the same version of software, and as such does or does not experience problems with memory leaks.
However, claiming such things do not occur with OS X and rebooting will not improve performance is ignorant and displays a lack of understanding how computer memory management works.
While IJ Reilly's problem is not likely caused by OS X itself, but by a particular program he uses, the simple fact remains that rebooting a computer, wether it is OS X, Windows, SunOS or whatever, can indeed speed up a computer significantly.
