If the definition of "Engage" is creating an immersive experience. If a core book were, and I'm not saying this is the case, nothing but crunch - is there an engaging experience?
It seems to me that you have to at least has some story and world flavor in order to even attempt to "engage" someone.
Am I missing a component of engagement when it comes to Crunch?
Crunch certainly can be engaging, depending on person and the quality of the crunch. If nothing else, some people get really involved in the mechanical side of things, beyond considering them a set of rules to be referred to. Having really good depth to the game mechanics can help this form of engagement. In some ways, 4E can be pretty good for this kind of engagement, with the complex but not immediately obvious interplay between many of the powers and mechanics. Character Optimization types certainly found ways to be engaged by crunch in 3E.
From a different perspective, rules and crunch are just as valid a way to present an immersive experience as story or world flavor. For a
game telling a story and creating an certain "feel" through rules and mechanics is essential, and a key aspect of what makes games different from novels or movies. Arguably, for a core rulebook they are far more important than anything way to present an immersive experience.
To relate some of what I mean... The absolute worst chapter of any book I have purchased from WotC was the Scientific Engineering chapter from d20 Future. That was something of a mess of a book no matter what way you look at it, but that chapter was far and away the worst. Why? Because it had no crunch. It just talked on and on about different ideas, and gave no real mechanics or even a suggestion of an implementation on how to use the ideas it talks about. I couldn't engage with it at all because it had no use to me. Anything mentioned in that chapter has no possible bearing on my experience with the game because it couldn't be used in the game.
On the other hand, if used well, mechanics can tell a great story even in the absence of other forms of storytelling. My favorite example of this is the D-Counter from the videogame
Breath of Fire: Dragon Quarter. Simply put, the main character in that game receives a power that is absolutely unmatched in raw might, but using it slowly kills him. In fact, just possessing the power means that he is going to die very soon, whether he uses it or not. And almost all of the "feel" of this certain doom is expressed through the game mechanic of the D-Counter, which slowly counts up until 100%, at which point it is Game Over and you need to restart from the beginning of the game. That mechanic alone tells half the story of the game, and sets a powerful emotional context for the story sequences.
So... yeah. For a game like D&D, story and fluff can completely fail to be engaging, and crunch can be very engaging. To be honest, I found some of the random references found in the power names of the Primal classes from the PHB2 to be more interesting and inspiring than more long-winded fluff pieces that filled many parts of the 4E books. That might be an issue of my one style of being engaged by fantasy and my own tastes in fluff, but I think it is relevant.