I am both delighted and honored to receive Dr. Hacker's correspondence--as well as the generous message of thanks left publicly by co-author Claudia Dreifus in the comments of the post itself--and given the opportunity, I composed a reply to them which clarifies and expands my earlier comments. What follows is a slightly altered version of these additional thoughts.
Firstly, I did not mean to argue that because many less prestigious colleges provide a great undergraduate education that therefore prestigious places which employ graduate teaching assistants do not. The PhD students in the United States I've met are brilliant, enthusiastic, generous people, and I feel fortunate to know them. Their undergraduates are likewise fortunate. So while I believe it is accurate to suggest that undergraduate education in the Ivy League schools is no better than it is in many other (occasionally unlikely) places, on the other hand I would be hesitant to argue that it is necessarily worse. Obviously, you do not need a research superstar to teach Sociology 101 -- nor do you need an instructor with thirty years of experience. Some of the most dedicated and effective teachers I've ever met are current PhD students.
Nevertheless, that fact does not justify the wholesale casualization of the academic workforce. My experience at Raritan Valley Community College was perhaps atypical. Like most community colleges, RVCC relies heavily upon poorly-paid adjuncts (some of whom are also graduate students in the region), but because I was taking upper-level courses as a student there I was fortunate to have taken classes taught primarily by full-time tenured and tenure-track faculty. I believe that this was an invaluable part of my experience. These professors provided not just expertise but also continuity to the educational experience. For students such as me, knowing that the professors would be there semester after semester, year after year, fosters attachment to the college and confidence in its mission. Thus the faculty was key to RVCC's strength. A strong community requires social stability.