This has been a debate with crew members for a very long time. A seasoned player would 1st check his skill details and discover that he has no exploration (specific) boost. So does his very presence on the ship offer a high point/strength value worth those (4) crew slots? Questions, but no real answers. It would be nice if @Lynsey [Midoki] can respond, but I understand if she can't. TiGeR
Is he not the one that is supposed to be good for taking on "Ghost Ship"? Or am I wrong? Or wait let me rephrase that. "He increases the chance of successfully beating Ghost Ship"...yes...am I doing this right?..
The Ghastly Gunslinger and Priestess have that skill, but unless I'm missing something the WD doesn't. TiGeR
Having a Witch Doctor on board does decrease the damage your crew takes during battles. Of course, if you're particularly unlucky, he may get chomped early on (I take a spare!).
I carry 4 witch doctors for an exploration voyage. They really help. Can attack up to 4 5 star ships/monsters per trip. So far, which has been almost 2 years of sailing with 4 docs, have yet to lose a voyage.