Most professors seem to get an itch, sooner or later, to write a textbook...
and many of them then proceed to require that same textbook for their
students.
If that's not a good way to stifle your student's ability to get more than one
point of view on a subject, I don't know what is.
Of course, many professors do have good books, I just think the savvier
professor would make his or her own book a recommended second book rather than
the primary one for a course.
I had a comp. sci. professor use his own text in a course that was best
described as, "Assembly language on a VAX." It was an OK book, I suppose, but
nothing to compare to the classic CS books out there. I do recall learning a
few things about magnetic tapes that I didn't already know -- something that
was about 95% useless already at the time, but still interesting. (I kinda
wish I'd had some EE class where they had discussed tubes at least a little,
rather than immediately relegating them to the corners of anachronisms...)