Mac OSX - Poor memory management?

One of the flaws I've see with the Mac OSX environment is that it's unable to manage its RAM properly. For example, after a few hours of use often involving online videos, multiple tabs, watching TV shows, the memory of my iMac or Macbook Pro eventually gets used up (possibly due to memory leaks) and forcing the smooth experience to become somewhat staggered by the lack of free RAM, while there is a huge percentage of unused RAM backlogged. The only remedy being to either restart or to use the Purge command in Terminal. Restarting it is my prefered method as the the experience is often more consistent after a fresh restart. I've certainly experienced this since Mac OSX Leopard, and with each reiteration I would hope it would improve.

In comparison I could leave my Windows laptops on for days on standby/hibernate, without restarting and the memory usage would still be performing relatively the same. Has anyone who uses both OSes on daily basis noticed this more often or even experienced the inverse?