I'll openly admit and even Lucas said so in the Vanity Fair issue leading up the release of Episode III, that he really only had enough story for one film. He's quoted as saying that "Episode I was 20%. Episode II was 20%. And Episode III was 60% of the backstory outline he had."
But, thematically, there are too many threads dealing with Anakin's arc that would've been gone had he just done one film. It's very clear, to me, that Lucas had alot to say/show with this Anakin story arc. And there are too many plots points that would've been lost.
I totally get the attitude of not wanting to deal with the "political junk" and the "love story" stuff. For many of you, it gets in the way of what everyone has been waiting to see. But, go back and reread the prologue to the Original Star Wars novel. It's the blueprint to the Prequels. On top of all that, Lucas in interviews in the early 80's, specifically stated that "if" he were to go back and do the "backstory", it would be totally different in tone, character, and era. He said he would go for something that was more about the "political machinations" of how the Republic crumbled.
I don't have the exact interview but I've read it plenty of times online in varies forums.
I totally understand the idea of "Episode III" is enough. I, in part, agree. But, I just don't think Episode III works withouth the threads he set up in the previous films. And again, that depends if you really want to see the "mother" and the "political" plot.
I like the fact that the characters, era, and tone is drastically different from the Original Trilogy. As much as he did copy the Original Trilogy, in terms of specific beats, if he'd copied the exact feel and tone of the Original Trilogy, he would've caught hell for not coming up with something different. As it stands, many feel that stylistically, the Prequel Trilogy isn't Star Wars.
All I say is why does Star Wars have to be only defined in one way? I understand the Originals started it all. That can't be taken away. But, why limit the vision and universe of Star Wars?