Why are the co-wives described as "exempting" one another from yibum, when really those potential cases of yibum are prohibited? One suggestion is that that language would have suggested a need for halitzah, and once there's a need for halitzah, there's a risk of presuming yibum was possible, when it wasn't. Another suggestion: Plus, the strong connection between halitzah and yibum. Also, the Gemara asks why these 15 cases of co-wives don't include 6 more stringent cases of illicit sexual relationships (which we know from the Torah, and will discuss those cases later in Yevamot). Plus, some discussion on the transactional nature of the yibum relationship.