I don't know if that many people inside the python community care either. Twisted is very big, documentation seriously lacking, and a lot of stuff in twisted not that useful if you only care about http. If you care about http, twisted is not that useful either (there is for example no http 1.1 support client-side AFAIK).
It would also have made porting tornado to python 3 a huge effort (given twisted code size).
I am aware of that code: it is undocumented, and does not have support for things as fundamental as keep alive. The underscoring does not inspire confidence either.
You're meant to use the public API twisted.web.client.Agent (which uses _newclient). If anyone's interested in finishing keep-alive support, the ticket is http://twistedmatrix.com/trac/ticket/3420
It would also have made porting tornado to python 3 a huge effort (given twisted code size).