Sure you wouldn't want to read a good piece of fiction at 500wpm. When I'm reading Wodehouse I deliberately slow down to savor the language and the dialog because it's so great. But the thing about Spritz is that you can choose your own speed. How about 300wpm? The point is that as a medium, it takes a lot less space (ideal for embedding on web pages etc) and for a lot of people it's an easier way to read. Sometimes my brain will inadvertently read a word on the line below and it will sort of "color" what I'm reading on the line above. With Spritz you have way more focus and stability which may improve comprehension and retention.
But yeah, for texts you have to "get through," 500 or even 1000wpm sounds great. Were you referring to "Code Complete" Craig? I've yet to get through that one, lol....