Many theology textbooks have an accompanying Reader volume, which are both boon and disappointment.
The reality is that students rarely read the textbook; they're really unlikely to read the reader. Further, these brief snippets are hard to get into. It becomes a chicken-and-egg scenario, as you need a context and background to understand an extract, and with a Reader's samplings you will never get a rich background to a particular theologian. I had to read Balthasar for years before I felt like I began to understand him. With brief samplings of theologians, undergraduates are lost in a contextless, backgroundless sea.
And yet, Reader volumes can give a brief soling without too much baggage. I don't know any other way to get an overview of millennia of theological material besides something like a Reader. Additionally, many Reader editions have introductions and summaries from the editor that help you understand the background and points of the writer and text. For graduate level students and beyond, a Reader volume is a great way to sample the endlessly diverse amount of theological writings that have developed.
The problem is unavoidable, that we must decide as instructors whether we want our students to know one theologian or theological writing very well, or whether we want them to have a sense of the vast scope of theological writing. Either way is insufficient. Personally I'd rather the beginning student have a sense of the scope, so that she can then go on to get depth in higher level courses.
But the truth is, we need both. The scope doesn't make much sense without the depth. We should probably make graduate students retake introductory survey courses are having mastered some depth. This is why that early teaching, after a doctorate, is so crucial, as it forces the instructor to add breadth to the depth that is already there.