Bingowings said:
Just as Obi-Wan allows himself to be killed he claims that if Vader strikes him down he will "become more powerful than...(he)...could possibly imagine" all Obi-Wan does in spirit form is give Luke a bit of shooting advice, direct him to Yoda and confess to fibbing a bit.
That's very true. So now the question is, how can Obi-Wan be shown more powerful as a ghost? I don't think it can be done, but someone could have an idea.