The whole idea is to help you be able to use Trollworld and incorporate it into your own campaign adventures. Having half the world unmapped gives you that chance to put your own place there and at the sane time, be able to visit the classic T&T continents when it suits your needs.
I recently finished a couple of maps that give close-up views of parts of the R'rrrlf that had previous not been seen at this scale and one of those maps takes place in a slightly different Trollworld reality. More about that soon...
Personally I hate settings that are all mapped out to the Nth degree. A) It gives me no room to do my own thing, B) It takes out some of the mystery and adventure. Quite often my characters don't know what lies the other side of the mountains, so neither do I.