I can't say with certainty yet (not having read it and unable to find my copy dammit) but the chief virtue of the silmarillion imo is that, like the 200 pages of appendices, it gives a historical background to LoTR, thus enhacing LoTR and giving it much more depths if you read it again. I admit that the movies really enhanced the books for me in terms of visuals (like the Balrog in Moria, Moria itself, the Argonath, the battles...) and likewise have the books made the movie better.
Btw, if you have grown bored of the Silmarillion, you could just send it to me and I'll kindly give it a new home ;)
:rofl2: