The Wizard of Oz is self-contained. The visuals of the film only need to meet the standard of the other visuals in the film.

Star Wars is different, in that there is a twenty five year technology gap between the first one produced and the last one produced. Comparing the creatures in the Cantina scene to even the creatures in Jabba's palace is quite a difference, let alone everything they were able to create for the PT.

Lucas' intention is for us to watch the films 1-6, because they tell one single story. Thus everything within that story is held to the standards of everything else within the story. I respect the integrity of the OT, and don't feel that midichlorians or the Force Ghost of Qui-Gon need to be added for continuity, because story wise I feel that the PT should have been matched better.

However, I do feel that to create a continuous story where you watch the films back to back, the visual difference between the films should be made less jarring. ROTS is beautiful on Bluray, while many scenes in ANH are very underwhelming, despite other scenes having been cleaned thoroughly. Take the opening sequence for example: The space battle looks great, engines and lasers are pristine but... in one place, you see a VERY outdated shot of the Tantive IV. Where is the harm in replacing this shot with a cgi model of the ship, to keep that single shot from standing out like an eyesore? The same goes for the scenes of the droids walking through the desert, where rainbow effects are obviously detracting from the picture quality.