AFIAK, Lucas actually wanted Qui-Gon to appear as a ghost and have a conversation with Yoda.
Then I think he cut that down to just a conversation, so that all Liam would have to do is some ADR.
Apparently Liam couldn't even be bothered to do that, although he accepted to do a photo op for the Star Wars Vanity Fair cover.
That just pisses me off. The whole Qui-Gon thing to me is a really important part of the story, not having it there ... the film still works, but it removes a nice of layer of depth.