I have no virus protection on my Mac, well not on the OS X side anyway. I have downloaded hundreds of GB in programs, pictures, videos, you name it, and not once have I had a virus, my system has not slowed down one iota.
There are not many OS-X viruses in the wild, yet. But even though OS-X viruses aren't as common as Windows viruses by a large margin, they are still out there -- and they're multiplying. The more popular Mac's become, the more viruses you'll see in the wild. Perhaps it's not necessary today, but it'll become more necassery in a year or so. Either that, or people stop buying Macs so no viruses will be created to target them.
Mac OS X maintains itself. It runs its own little cleanup and system programs silently. You don't have to use spyware programs, you don't have to defrag, you don't need to "optimize" your system.
Mac OS-X doesn't maintain itself anymore then Windows does, generally. However, the user maintenance required is indeed a lot less. On spyware: even though viruses aren't all that common on OS-X, spyware is a different matter. But then again, spyware isn't usually a big deal. However, OS-X certainly doesn't clean it for you. Defragging is not needed, you are correct on that point, because of the filesystem. That doesn't mean OS-X doesn't clog up if you don't keep it healthy -- it certainly does. Almost any OS does.