I think General (or even Darth?) Grievous has the potential for a great temporary villain. I agree that it would be nice to focus on one villain during the trilogy and have that one being finished off in Ep3. But I have to say that I'm not entirely unhappy with the approach that is taken for the prequels. These films are not about those villains, they are about the guy who's about to become the one ultimate villain that outlasts all the others before him.
I think that it's not that bad to show that Sidious considers his servants to be nothing more than disposable assets, that only serve one purpose - to further his goals. It just proves that Vader is much tougher than all those guys we've seen in the prequels. Heck, he even kicked his Bosses' ass when he tried to replace him like the others before him.
It just puts the focus away from those guys and onto Vader, if you ask me. I like that alot. All power to the dark lord. Forget about those three other wimps, they were Palpatine's fancy cannon fodder - but nothing more.