When I read a great book, I usually end up thinking, "Damn, I wish there was more about that one thing they briefly mentioned". Usually, if somebody goes in later and tries to fill in the blanks, you end up with a turd, because they're going in later to answer questions.
The Silmarillion is different. I'd argue it's the main work, and I appreciate it largely because it's not a novel, it's a Biblical compilation covering thousands of years that required decades of world-building and re-writing to bring it to the incomplete state it's in now.