I really don't think that if DF went open-source it would be endanger of being cloned any more than it has already been (Towns, Gnomoria, etc; all very much DF clones, albeit with a smaller scope and better graphics). If you knew enough programming to develop a game, you could easily duplicate DF simply by playing it and reading the wiki to learn how everything works, mechanically and then figure out your own way to code it.
From what I've seen Toady say about the game and his work on it, I would think that the main reason it's not open-source is because he likes working on it. It's his baby, and it also gets him attention. If he was to give out the code, Toady would probably see a lot less personal attention, fewer people would follow his original programming line and would splinter off into different mods, etc. Still splintering the fan base, but at a bigger cost to his "fame" than the way current mods split the community.
Money would be another thing. Game is free, but powered by donations; since he's the only one able to update it, it gives him more "power" for receiving donations. If it was open-source, he could potentially lose more donations as people move to a different development branch from someone else, who may also start accepting (and receiving) donations for their updates to the code over Toady's.
Personally, I don't really mind if it's open-source or not, but I would like more modding abilities. There's a lot you can do, but also a lot you can't. You can make new items, new creatures, new buildings, etc, but you can't really add new features; only change the names and stats of existing ones.