That is the part that I've never been able to figure out. The unification types seem to intellectually realize that North Korea is a bad place lead by bad people. Emotionally they seem to think that Koreans would never really want to hurt other Koreans, and that reunification would solve the North / South differences.
"If only Korea was unified, then the war would end, the Americans would leave, and the land would be healed."
A fairy tale, perhaps, but after living for fifty years under the threat of war and death, an understandable one.
As you said, there is a strong latent desire to solve the division of Korea which was imposed by big powers.
The left use it to ignore the brutal oppression in N. Korea. To them, the division is worse than the details, such as the brutal nature of N. Korea. The left is appealing to the twisted kind of nationalism or patriotism, which is that standing up to big power matters over all other considerations, even if it means the wholesale oppresseion of one's own kind.
The left made it politically correct thing to do. The closest analogy is Israeli's accomodation at all costs under Barak gov. The accomodation was PC in Israel until the Temple Mount riot started the current series of unending suicide attacks.