Well, I'm an end-to-end reader for the most part, but usually the delving is what attracts me to a book if I am looking in a shop. I'd say the downside to the end-to-end approach, for myself, is if I am put off by the first part of the book it is very likely to be put on a shelf and forgotten about. Wheras if I delved and found something interesting right away, then expanded my delving aound that, eventually all the pages would be accounted for anyway, and the thing wouldn't have been on a shelf for two years!
As I write this I am struck by the contradiction of being an end-to-end reader, while in real life I find the shortest distance between two points is the longest, seemingly most random path between them. Hmm.
I generally keep books I've read as I like to go back to them.