True. Not everyone who watches the prequels has necessarily seen the originals or been exposed to the EU. In fact, I have to admit that even I haven't read/seen a good chunk of the expanded universe stuff out there, aside from a few Star Wars EU books and games here and there (In a game of Star Wars Trivial Pursuit I wasted my friend in the trivia from the movies, but he had the upper hand in the EU trivia).
There is a lot of EU material out there, and one of the things I don't like about the EU is that there are a lot of plot inconsistencies. This is understandable (even excusable) because there are so many different authors/game designers/producers, etc. that it would be impossible to have every book, tv show, game and comic connect perfectly. For that very reason, I don't consider the EU as having any authority on the actual storyline Lucas wrote. In other words, I don't think the EU has any bearing on the story of the two trilogies. That doesn't mean they aren't good stories, however.
What I don't think is excusable is how George Lucas, who is the final authority when it comes to the movie story lines, failed to properly connect the two trilogies together. It's like they're loosely based on each other (like the EU) but there are still plot inconsistencies between the two trilogies.