It's a fine option, but over, say, 30 years (well within the lifespan of a good textbook if it's getting updates) there are likely to be copies of the book that

It's a fine option, but over, say, 30 years (well within the lifespan of a good textbook if it's getting updates) there are likely to be copies of the book that aren't accompanied with the source code. Especially for books where instructors might want to make printed copies available (less so for my books, but definitely a possibility for textbooks for 300 student introductory classes). IMO the FDL makes that scenario less likely and so strengthens the project which is worth the added inconvenience of the license. The FDL also allows commercial use and textbooks can be expensive; I think it's likely that a publisher would try to bundle or repackage a free book if they could make money off it, and I don't think they're too likely to make it obvious that the book could be downloaded for free unless they're required to. CC NonCommercial would be less free than the FDL; I don't mind someone else making money off the books per se (especially if they are providing real value, but even if they're not), but a lot of the strategies that many textbook publishers use to generate revenue don't serve students very well and would weaken the project. Like I said, I appreciate the effort put out by people making educational material CC, and I appreciate it even more if they make the source available. But for this type of project I think the source being available is the most important thing so I've chosen a license that makes it mandatory. For other projects I might decide differently.