mrjohn, I'm not saying KeywordKeyword.com is equal to Keyword.com - I'm just saying these domains are easily memorable (people need to remember only one word, just like Keyword.com domains). The fact they have been regged for years (and keep being renewed) is another indicator. I think they definitely have a reason to exist
My thoughts . . .
1. SERP superiority, at the end of the day, is the name of the game. Someone, even if it is just the end user you sell the domain to, has to have reasonable confidence they can perform well in Google with the domain.
2.a. Regged for years doesn't mean anything. I've seen absolute gibberish domains that are still regged and undeveloped for years.
2.b. Lots of sites come out of nowhere and built an audience in less than a year.
3.a. For me to even care about a namename.com, I'd have to see significant failure to develop that keyword on other domains.
3.b. Off the top of my head, I can't answer what Matt Cutts would say, but if I had to bet, Google penalizes them in SERP. It just seems too obvious and too domainerish for Google to not punish. Google never passes up a chance to deliver a face slap to anything that has easy blackhat SEO potential.
3.c. If there is failure to develop the name.com, name.net, name.org, etc. domains, you have to ask why.
3.d. If there isn't failure to develop them -- that is to say there are well-SEOed sites score good SERPs for the keyword -- then what's the point? What's the prize?
4. Saying something doesn't make it true. Too many domainers want to use the magical wish-granting pony track to instant wealth. It doesn't work. Namename.com domains are a magical wish-granting pony. Nothing more. If you want to put your money in a pile and set it on fire, go wild.
5. The only ones that would work are ones that work in a sing-songy sense. Like LoveLove.com, MomMom.com, DadDad.com, etc. TabletTablet.com?
6. Memorability is over-rated. Remember, people go to sites with names like ArsTechnica.com all the time.
I'd rather put my time and money into link building (if fairly priced), SEO/SERP and analysis.