<p>Many universities use grad students to teach intro language and intro math classes. It would be a waste to a have a professor teach those classes. The use of grad students in those cases can help keep class sizes small and allow offering of classes during more times of the week. The biggest complaint I hear in that situation is that some grad students are hard to understand because English is not their first language.</p>
<p>I understand that US News ratings penalize colleges that have large numbers of classes taught by part-time instructors.</p>