Certainly Belew does infinitely more complicated stuff than I could ever do, but I too am lacking in my ability to read music and have to be sure to memorize whats coming up next in a song, or at least hope that the music itself will lend me hints as to how it is progressing, by ear, and hope I can keep up. A lot of "complicated" songs I have been a part of creating with other musicians are usually pre-determined patterns with slight variances. Once you know the pattern inately, you then can modify it depending on what the song calls for. At times, songs that seem wildly complicated are 4 or 5 main patterns with variances in each, creating the illusion of a "monster" arrangement of 15 to 20 sections.
I have noticed, however, that a lot of songs I DIDN'T write have the same formula: Variances on a pattern(s). And when done properly, it makes for a very dynamic song. (I am not speaking of a 12-bar blues pattern here...I mean patterns like the song in question "Frame By Frame" which, although its a complicated and hard-to-play pattern, is nonetheless repetitious in the end.) When done poorly, it begins to grate and can sound seriously lame. (enter 12-bar blues again)
Few musicians can make genius arrangements of long, complicated songs truly unique, never repeating, and highly rhapsodic without having to rely on pattern variances. Those who can do this, of course, tend to be able to read and write notation as easily as reading language. One insane example in the jazz world is Michel Camilo, whose over-the-top jazz instrumental arrangements would send even the most seasoned musicians crying for mommy. Thats why his trio is himself, Anthony Jackson, and Horacio "El Negro" Hernandez....I mean, who else?
My wife can sight read, but I've never gotten around to having her help me learn. Oh sure, I can read a guitar or bass tab with ease, but my notation reading easily less than 5% of what it should/could be with the right effort. Which I don't make, sadly. Its the biggest, gaping hole in my music theory knowledge at this point. I used to even give guitar and bass lessons once....had to skirt the subject of notation on many occasion. The embarrassment!!