While this is a partisan comment, it highlights the political reason why the DNC pursues registration drives. It stands to benefit from doing so. Voters in demographics more likely to support the Democrats are less likely to register and less likely to be engaged.
It should be pointed out that the various organs of both the Democratic and Republican party both run voter registration drives, as do a number of other groups (with varying degrees of affiliation with official parties).
It's not surprising that each group targets demographics which seek to further their aims.
Despite that, registering more voters is categorically a good thing. We as a society require an informed and engaged electorate. Registering to vote is part of that engagement.
That's only because the GOP have decided not to cater to the vast majority and instead are held hostage by their tea partiers/religious right/corporatists.
Democrats have their share of right-wingedness, but also cater to marriage equality folk, minorities and progressives.
When Republicans give up on 47% of the electorate, it's a clear sign that, for them, voter suppression is more effective than registration.
