Ordering books for college classes is its own little world. Calculating the difficulty of the readings, the length of the readings, and the cost of the books is a challenge. (I refuse to select at textbook that is more than $90. This is getting harder to do.) You strive for balance in readability versus challenging the students, enjoyability and applicability versus authenticity and depth.
Textbooks make this all easier and harder. With a course textbook, the course cost for the student goes up significantly (at least most of the time), while the textbook is free for the instructor (a nice incentive to go with a textbook). Most textbooks give great overviews of the material, and there is a simplicity in having one book to rule them all--there is no confusion about which book to read or bring.
Textbooks are also great for showing how ideas play off of one another, as a free-will reading can be directly paired with a determinism reading. This gives the issue a sharpness that exceeds that of reading one particular thinker, who would most likely take stand and move on, rather than revealing the debate at hand. Especially in a field like philosophy, where there is so much variability, this can be quite illuminating. Textbooks also reveal the scope of a field. A theology textbook must cover 2 millennia of history, ideas, disputes, and changes, and it would be difficult to achieve that kind of scope with only a clutch of monographs.
Yet many humanities textbooks are, by their very nature, extracts from other books, so that the students never get a real in-depth plunge into a single writer or point of view. They are given samples of various authors, but don't really get into the mind of Plato or St. Augustine. Everything stays fairly mild, in some ways. Yet, for an introductory course, is this not a benefit? This is where I stand now, at least, so that instead of getting creative and ordering primary texts, we are using some textbooks.