My course 8 and 18 recitations were almost always taught by faculty. Course 5 often uses TA's. It is true that for most students, the class in which they are most likely to have a TA is a freshman recitation, it is also true that the class in which they are most likely to be taught by a Nobel laureate is a freshman recitation. A couple of MIT's most storied faculty, who could teach ANYTHING they want, or indeed nothing at all, choose to teach freshman. Part of this is the ability to "make the sale" to students choosing a major at the end of their first year along the lines of "I chose to devote my life to Physics, and let me try to convey to you why that is." Storied professors who don't have Nobel prizes also often teach freshman. For example, Professor Eric Lander (the first author on the publication of the human genome) normally teaches freshman biology.