I would LOVE to use free textbooks and older books. I try to do so as much as possible. But the real problem (for me at least), is how can I get a copy for all of my students? I can't afford to print them all a copy of old stuff, and after about 5-10 years, the publishers stop printing the older books!
I came across this problem big time this school year -- planned on using a book I love and have been using for 10 years old to be told last minute it's now OUT OF PRINT. (It was in print just 6 months ago). sigh
Old is OK in many places, but only if you can get 50-100 copies of it each semester / year...