Democracy means having some say in who your elected officials are. In some parts of the country, the only way to do that is to register in one party no matter what your personal views.
I live in a one-party corner of a one-party state. 85% of our state legislators are Democrats, a number unmatched elsewhere. Only the Idaho legislature comes close in being as strongly Republican. I registered as unenrolled because Republicans don't bother to field candidates in most local elections and I see no reason why they should start; the conservatives, such as they are, run in the Democrat primary. So I can take that ballot when I see a choice worth making.