The problem with that is... it works until it doesn't. Memory leaks, instability, SLOWNESS and even data corruption can sneak in if left on long enough. A reboot is your friend.
Ah, the Microsoft view of the world, yes? No need to fix memory leaks; just reboot frequently, and everything is fine!
No, OS X is based on BSD. In the Unix world, a memory leak is treated as a serious bug; it is something to be fixed post-haste, not to be ignored. And OS X is, indeed, a very solid OS.
The apps I choose to use are also solid -- Thunderbird mail, Chrome browser. I've been surprised to find that even iTunes, an application I would have expected to be buggy as heck, works fine even after being up and running for a month, without any obvious memory leakage.
OS X is not Windows. Badly written code should not be the standard here. If you are experiencing memory leakage, then something is wrong!