the free level ups were designed so that you wouldn't have to do korthos... granted you still don't have to but its a good way to get favor (esp coin lords for the bag slot being the most useful item at any level)
im guessing almost everyone who does the level 4 advance will go back for some missed quest rewards and favor (i went back for +1d4 handwraps too)
so despite being able to skip the whole island there i am back with that annoying "oh no the suhaugin have found me" guy since they didn't provide korthos island favor