As BM said I think it is very much of it's time. Stoned hippies driving around in a bus waiting for things to happen wasn't a new idea - the Merry Pranksters made the concept famous as early as 64, as Paul, with his keen eye on the underground scene, must have been aware.
The lack of script, plot and characters was also very sixties. It's reeks of the era's "we can do anything" ethos. The Magic Christian isn't too far removed from this.
Had it been put in the hands of professionals it might have turned out as nothing more inspiring than an episode of The Partridge Family.
If commercially it wasn't the greatest idea it was at least one more idea than anyone else in the band was having. I think Paul deserves plaudits for this brave little stab at arthouse.