I'll tell you why I bought them and feel it was a smart move. Because Darkcoin is superior to Bitcoin. It answers the needs of the current primary users of Bitcoin (black market) better than Bitcoin does, and it has a more rational production curve than Bitcoin does. This is a perfectly free market, with no regulators picking favorites, so it seems to me essentially inevitable that, at this early stage of the e-currency game, a manifestly superior system will beat out the others.
Who knows how long it will be until ordinary people start making the switch in order to defeat their condition of abject enslavement by the state, but when they do, between a currency that has a built-in anonymity feature and one that doesn't, the former wins. It is the right horse to back.