Pure democracy is a bad thing - one swift kick away from ochlocracy, anarchism and already is subject to plutocracy and popular rule.
Effectively - no one is equal, some are more popular, some are richer and some will always be more influential, basically destroying the concept of a democracy all together.
Actually this isn't a bad idea, a civilisation with pure democratic tendencies will eventually evolve into one of those mentioned above. That is if they don't regulate it like Sparta and Athens (Exiling those which became too popular and influential).
Pure democracy is not a bad thing. It simply
cannot exist because in order for it to exist people would have to take reputation into no account and have completely accurate information about everything in the society in order for each individual citizen to genuinely make a decision rather than being someone elses political cattle.
There is no reason however that the appearance of a democracy would not be mantained indefinately because those who are really in charge (the political cattle-herders) find it conveniant that nobody notices that power actually descends from them, so their opponants will leave them alone and only bump off the herd-leaders, whether by electoral or other means.
Any state of affairs where the government actually controls the information and the economy, that is a state of affairs where those who hold the supreme power are the official rulers is stamped as a dictatorship using their own undemocratic power and democratic ideology. Thus 'people' are herded overthrow it in a show of democracy which is actually a civil war betwen two rival groups of elites, the 'public' elite and the 'private elite' neither of which are the mythical People of Democracy.
Given the communist nature of the economy in Dwarf Fortress, there is no illusion of democracy possible because the government's economic mechanism determines the ideological production and it rationally does so in order to glorify/strengthen the government in the eyes of the people. A 'free and fair election' is not possible because the government controls the information about itself meaning that is will always be able to pursuade all the individual people to vote for it.
All opposition is internal to the government, meaning that there is no real opposition party to contend with the government just loyalists and rebels. The ability to support an openly manifesting rebel faction within the ruling party if there is one is something that exists within a dictatorship.
Also, on an unrelated (and for the most part purely academic) note, since we're dividing governments up by the number of people ruling, what about governments where there is no ruler, not in the sense of anarchy where there is no government either, but in the sense of trying to establish a pure theocracy in the real world, a government where the ruler does not exist (or is completely absent and detached to the to such a point as makes no practical difference from non existence)
Anarchy is actually essentially non-functional in the game (except for kobolds apparantly), I have actually tried creating one. The reason is that without anyone central dictating military goals the civilization cannot ever make peace with it's enemies. This means they continue to wage war until they are wiped out; but they cannot ever go on the offensive without a central government to command them to do so.
It all makes sense though when you think of it. If there is an anarchy and a government side by side, the government will always ultimately crush the anarchy because there is nobody to negotiate with that can actually command the others to abide my any treaty or agreement about the borders of the state. At the same time there is nobody to command them to organise the occupation of the government's territory, because if a small band of anarchists decides to independantly kill off the government in a place, a new government can be created to rule that place when they go home.