I have been an instructor of freshman English language and writing on a college level for several years. While each college had its own required textbooks for the course, I have always supplemented them with Merriam-Webster's Vocabulary Builder. In the beginning, the reason was two-fold: first, this great little book gave students the chance to learn and use, correctly, new words to help them become better readers and writers; and, second, because it allows students to appreciate the vast pool of influences from which the English language is drawn. Students also appreciate that all this is available in a book that costs them the price of a mocha latte.
However, a recent development has added a third reason for making Merriam-Webster's Vocabulary Builder a required text in my class: technology. Too many new students feel it's appropriate to use "LMAO", "IMHO", "BTW", etc., in their essays. It is not appropriate. Of course, abbreviations have long been a part of our language, but this shorthand is getting students away from real words and their meanings. To me, however, the biggest danger comes from the thesaurus software that is available on every computer. At best, students use alternate words without learning or retaining their meanings. At worst, students are using these words, thinking that they express what they really want to say when they do not. For example, one student used the word "myriad" instead of "many" to describe the number of papers she had to write for a particular class. Yes, she may have had to write MANY papers but not a COUNTLESS NUMBER of them. (The student was not being creative or indulging in hyperbole; she thought "myriad" meant many because the thesaurus offered it to her.)
One final comment, students actually like the book. The affordability aside, several have mentioned that they like "the feel" of the book, that it has a certain "authority" while also being accessible. These reasons may sound NEBULOUS but it's the type of feedback I've received over my MANY (not MYRIAD) years of teaching.