About CGI freeing filmmakers from constraints, that is true, but it seems to me that Lucas is using it as the easy way out for him. I was just watching some of the documentaries on the Episode II DVD a few days ago, and I was really surprised to see that in some of the scenes (don't remember exactly which ones at the moment), he didn't even take the time to build sets, he just had the actors in a green screen room and then added the set using CGI later on.
What I don't get is why Lucas doesn't combine old and new technology for special effects. For ships fighting each other in outer space, he could easily use models for that and then just enhance it with CGI when needed. That would make it look much more realistic, because models ARE real. When it comes to outer space battles, I'd pick the original trilogy ones any day because they looked realistic, while the prequel trilogy's battles look fake a lot of the time, because you can tell that it's made from computers.
So CGI might make things easier, but a lot of the times it's just used by filmmakers as the easy way out when there are lots of things done in CGI that could also be done with models and miniatures and stuff, which would make it look real. But I guess filmmakers still haven't gotten over the fact that they've got CGI. It's like when you get a new game, you keep playing it alot over and over again, but as time goes on you eventually stop using it so much and stuff after you've gotten used to it. I think the same thing's happening with filmmakers, and Lucas in particular. He's just gotten to use CGI and because it's so new he's using it as much as he can because it's so easy and everything. Based on some of the sequences that are in the prequel trilogy, it wouldn't surprise me if Lucas made some of those as big and huge as he did just to show off what CGI could do, and just because he could with this new technology.
Well, hopefully one day CGI can be perfected so that it looks completely life like, but until then models, puppets, miniatures, etc. are always going to look more real, mainly because they actually are real and not something that comes out of a computer.