They did it to trap the rebels. They knew if they built a second Death Star the rebels would come and try and destroy it, giving the Empire the chance it wanted to wipe out the Rebellion once and for all. Think about it...the rebels knew the devestating capabilities of the original Death Star, so the prospect of a second basically leaves them no choice but to try and take it out. Plus, if the Empire were to build something new, the rebels would have no idea of it's capabilities and would not be afraid of it, and thus would not rush to destory it. The point of building a second Death Star was basically just bait to lead the whole of the Rebellion into the well laid trap of the Emperor. And it almost worked...however, they weren't counting on the rebels being aided by hundreds of little furry jungle fighters.
And to add my 2 cents about some of the other ROTJ "flaws"...
1. Stormtroopers being "elite" but being defeated by Ewoks?
When were they elite? They ran in fear of Han Solo in ANH, why?? He just ran screaming at them and they fled! And about them being clones? If that where really true in the original concept of the Star Wars universe then how is it possible for the Emperor to have an "entire legion of his best troops"? If they're all clones, then there's no such thing as "best troops"! Just another example of Lucas trying to cover his ass.
2. Empire underestimaing the Rebellion.
That's obvious. "Your overconfidence is your weakness" "Your faith in your friends is yours!" Luke (and even the Emperor) knew that the Emperor was over confident. (well the Emperor thought he was very confident, but not over confiedent). He even refers to the rebels on the Sanctuary Moon as being a "pitiful little band". How was he to know that with the aid of the Ewoks (who I have always liked, I might add) that they would be become a formidable force?
3. The Empire surrenduring after the Death Star is destroyed.
The Death Star wasn't the only thing destroyed. Star Destroyers, TIE fighters, "an entire legion of the Emperor's best troops", plus the Empire's flag ship, the Super Star Destroyer. Not to mention the death of every major officer in the army (who were either on the Super Star Destroyer, Death Star or one of the many Star Destroyers), PLUS the death of the Darth Vader, second in command of the Empire, PLUS the EMPEROR, the figurehead and dictator of the Empire itself! I'd say they took a pretty big blow, wouldn't you say? One they wouldn't be able to recover from, you think? Plus, the Rebellion had some pretty strong support. That's why they had the best fighter ships in the galaxy being made exclusively for them (the X-wings and A-wings). Once the bulk of the Imperial army was wiped out, it wouldn't have taken much to overthrow the Empire.
I've always found it very difficult to say which is better of the 3. Ever since I was a kid, I've liked ROTJ the best, but that's only because of the speederbikes. I LOVED those bikes. Now that I'm older, I think I can honestly say it's a tie between all 3. I like each for differant reasons. Honestly. I don't think any of the 3 is "weaker" or "stronger" than the other. Each has strong and weak points, to be sure, but that doesn't make one better than the other. ANH was the intoduction that blew all our minds. ESB was emotionally heavy. ROTJ was big on action. They're all amazing, in my opinion.
Don't listen to Jimbo. He's like, 16 and doesn't know what he's talking about. He also thinks that Attack of the Clones is "by far" the best Star Wars movie... *rolls eyes*
And in response to something someone said above earlier...
Although I've never cried when Vader saves Luke (it is a powerful scene tho), I have cried when that one Ewok dies from the laser blast and the other tries to wake him up. (Come on! Haven't we all?? It's SO sad! The poor little guy!)