I had to come up with a T-shirt logo for our cub scout pack. I'm not a graphic designer by trade and not enough of a hobbyist to justify purchasing an overpriced piece of software from Adobe. InkScape and GIMP are more than enough for my needs and probably meet the needs of the majority of folks out there.
Just like Microsoft Word hasn't added any must-have features in years, I'm sure that the additional functionality present in the Adobe products caters to the niche markets that are willing to pay a premium. Since I'm not overly concerned with CMYK or whatever the equal-tier feature is in Illustrator, a book like this might just be what I need to get more use out of what I have available.