Yeah, it seems like there are multiple uses for rulebooks:
1) the (often initial or first few times) read of players rulebook that gives you inspiration for different kinds of characters you might want to play, a sense of the rules, etc. Once this read is done, it is often internalized and never read "cover to cover" again.
2) a reference manual to create characters and reference rules. -- ongoing use .
3) purely to read -- for various reasons the person will never play the game and just reads the book to gain ideas, make characters in their head, because they like to see new game mechanics, etc. (I often wonder if this is a pretty big audience? Not for D&D, but I am this person for many other systems.)
Game book designers are writing for all three purposes and some games emphasize one over the other. Sometimes on purpose, sometimes because it is hard to balance all three.
4e PHB defintely leaned 2 but I don't think completely ignored 1. It probably wasn't as good for 3 as you wouldn't get the ongoing benefit of the conciseness of powers, etc.
PF adventure paths actually lean 3 I think but of course are also played by many.