[Rules] help 'inspire' things you might not create on your own. — Fang Langford
I always intuitively felt that D&D, as a game of creative imagination, was intensely flavoured by its rules. I didn't really understand what this intuition meant when I was a high school–aged DM and I was trying to figure out why I did and didn't like certain sets of rules. Later when 3e came along I bemoaned the treatment of psionics, but I could never articulate why beyond complaining, "it's just magic with a different paint job".
Which brings me to the monograph I've quoted up there. I loved reading game rulebooks. Like a sponge, I sucked up the ambiance that the art, layout, side-bar fiction, examples of play, lists of equipment, and the mechanics themselves wove together. Most importantly, reading game books made me want to play because my brain was overflowing with scenes of such delightful events and imagery. I wanted to pull those out, show them off to my players, and then see them inhabit, explore, and expand these imagined places and histories.
I'm sure that this is why I was never a fan of GURPS and other "universal" systems. They deliberately omitted the very inspirational elements that resonated with my imagination. I knew that much of the reason for such universal systems was for them to impassively and impartially represent any imaginary world I could think of, but they were dull, dry, and soulless: I wanted them to sing to me, and they just gave me a blank staff. Sci-fi almost always went with dry and mechanical rules too, which would explain why none inspired me until I became aware of Blue Planet and Shock.
There have been a lot of arguments lately that rules don't matter, just the roleplay. Mostly I've seen this argued by people advocating for D&D 4th Edition against hold-outs. Even leaving aside the inherent contradiction in that approach ("you should use these rules, because it doesn't matter what rules you use"), there's a problem with this line of reasoning. The rules do matter. The fictional objects they emphasise; how they feel and flow as they're handled during play; the implicit and explicit bounds on the fiction that they represent: all those serve to inspire the imagination in different ways than another set of rules would.
The roleplay is certainly paramount, but it doesn't spring from players in a vacuum. If the roleplay that a group produces can be likened to a meal, then the game rules are an important ingredient that contributes to the overall character of the final dish.