Proxy settings are 'manual routing' on the application layer of the protocol
stack. Therefore proxy settings must remain a per host property in any case.
Even if most proxy users will only use one proxy setting for all their
connections, we must not restrict HttpClient to this most common case. I think
it is an essential flexibility to be able to use a specific proxy per connection.
At our company for instance, we have access to the intranets of some of our
clients. It makes sense to set up one proxy server for each of those
connections. With HttpClient I can then access any host in those intranets
depending on the proxy setting.
Also, if we ever want to implement PAC (auto config) we need to be able to use
one setting per host.