I think he mostly did it "because he can."
He probably thought "hey wouldn't it be awesome if I could show the movie and then say to the collectors who wanted to buy a Clone uniform.. 'but there was no clone uniform costume, they were all made in a computer!' and have their jaw drop to the floor."
Lucas probably lives for such moments.
Actually in ROTS I thought "these guys look so REAL" until I watched ANH on DVD again barely 24 hours after my first viewing. And then I saw what REAL stormtroopers looked like and realized how fake the ROTS troopers had been. It was just because I hadn't seen the originals in awhile that I didn't have a fresh, crisp picture of them in my mind to compare.
They were a little too fuzzy and plastic, vs. the shiney, gritty, grimy, clanking outfits.
He could have just gotten a bunch of guys of similar heights and used shoe lifts and camera angles to produce the rest. Even crowd scenes can be done without CG, ever hear of blue screen techniques? Those weren't exactly a novel phenomena even back in the late 70's, early 80's.
Like somebody said, film a bunch of guys and then composite the rest in.
Actually in the OT there are several crowd scenes where they just used cardboard cutouts (or mattes) which appear to be giant photographs of crowds. They look totally real until you take a still frame and look carefully, then it looks like crap. And none of them were fixed in the Special Editions (except for the ending of ANH).