It's called luxury marketing. Read it up. They're still doing it, btw.
You give something to the market. Something exclusive (something that excludes us mortals). Like a 10K TAM. People crave it. They won't buy it though. But they can still buy a Mac. It won't cost them a fortune, although it's still going to be more expensive than your poorer neighbour's beige PC.
They're still doing this, as I mentioned. The PowerBook is something exclusive. It's not cheap, in fact it's among the top end (price wise) notebooks out there and has a style that makes people crave for it. They (the mortals again) probably won't buy it, but they'll buy a piece of the dream of owning a TiBook when buying an iBook.
So, although the 20th A. Mac - of course! - didn't make sense in the sense that everybody would buy one, it TOTALLY made sense for the marketing. When it was introduced, magazines like Vogue, magazines like Playboy, magazines like WIRED, magazines like GQ they were all drooling for the item. And made their readers drool. And the goal was to sell more lower end Macs.
I'm certain, the Cube had a similar effect, although I'm not sure that this was the exact goal of Apple there. (I guess Apple really thought there was a need for smaller towers and less need for expansion.) I know several people who were totally Windows. They came to me saying that the Cube was cool, that the Ti was cool but that they were too expensive. And they ended up buying iBooks and iMacs, one even got the last Ti they had in a store, as he got it much cheaper.
So. This just to explain why it made sense to produce a limited edition totally-high-end 20th Anniversary Macintosh. And why it would make sense to have such an item in Apple's line again, maybe for the 30th?