<p>Funny, at my alma mater, TA's are limited to tutorials (and in many cases, like for example Calculus, are often better appreciated than the actual professor teaching the class, well at least some of them). A graduate student actually teaching a class gets the title of Instructor, while people with Master's or PhD without the title of professor (including assistant, associate, adjunct, visiting, emeritus or otherwise) who teach classes get the title of Lecturer...</p>
<p>And trust me, not all professors like teaching the low level courses, some take it almost as punishment, while a grad student is more likely to take it as an opportunity to get teaching experience for his/her resume, and will be much more eager to do a good job to get good reviews.</p>