I've only played DoW once coop. PLaying coop mode solo would be doable, but honestly, I don't know why anyone would bother.
DoW/TLN is very much about the players at the table and the balance of selfish vs selfless, lying vs honestly, manipulating the other players, etc. Taking that out of the game is fine and works OK, but you are missing a good chunk of what DoW such an incredible game IMHO. You lose a lot of the tension and gamesmanship.
So if DoW is a game you are looking at buying but MAY also want to play it solo, sure, you can do it but it's not as great as the actual "real" game. If you are looking at DoW more as a pure cooperative game that you would like to play solo on a fairly regular basis...meh. I may be tempted to say you should look at other pure coops.
Some people may only be able to play solo. But they still want to check out the game.