There's another good reason not to use teleology to assess humans: we know what tools are for, because we made them, so we can criticise them with reference to their purpose. Even if humans were created for a purpose (which I don't believe) we have no access to that purpose. This leaves the field wide open for interfering busybodies to tell us what we were made 'for' and to criticise us because we don't meet that function. I find theologians and psychologists falling into this trap. Evo psychs can be as bad as fundamentalists. But they all have one thing in common - they are most confident about the 'true' function of women, and very ready to criticise us for not meeting our proper function.
What has prompted this outburst? Well, Leon Kass has posted a second essay about how modern women are the pits, and how society is in trouble because we got uppitty ideas. And here is teleology right at the fore of his argument:
'Thanks to technology, a woman can declare herself free from the teleological meaning of her sexuality — as free as a man appears to be from his... Woman on the pill is thus not only freed from the practical risk of pregnancy; she has, wittingly or not, begun to redefine the meaning of her own womanliness.'
Please bear in mind he thinks this is a bad thing.
OK, my first reaction is 'sod off'. The second is - there is no meaning of my womanliness, imposed on me by a big daddy, neither Leon Kass nor his representative in heaven. It is not a sin for me to define the meaning of my own life. Having (for instance as I have done) a hysterectomy does not sever me from a teleological purpose. Because I create my own purpose. I am my own purpose. And of course so are you.
This same teleological fallacy is used to attack homosexuality (just as one example). There is no true purpose of human sexuality, imposed by either God or Nature. Legs were once fins, wings were legs, neurons were originally skin cells. Original function is meaningless in construing whether a current action is good or bad, god-given or sinful.