In my experience there's a lot of layout that's been done in Illustrator. I suspect it's because of how long Illustrator's been around and had pretty good layout support, especially when compared with the early years of Aldus PageMaker and QuarkXPress and other tools. Illustrator was needed to do a lot of graphics work that these other tools didn't support, so it can make sense to do as much as you can in a single tool. If you can get good results doing everything you need in terms of text handling in Illustrator, why take the time to use a DTP application when you're likely going to need to use Illustrator anyway for graphics work.