It's hard enough to find a dragon egg without having to worry about somebody else getting there first. 
Seriously, one reason to play dark at least once is the humour factor in some of the quests. Some of them are exactly the same as for the light side, some are very similar and some deal with the same objective from the opposite point of view. Of course some are totally different.
You also get to see some places you probably haven't visited before. Aren't you curious about what the wizard school is really like inside?
