Quoting Tim Wilson-Brown - teor teor2345@gmail.com:
If we ensure that Exits must pass some encrypted traffic, then running an Exit is less attractive to an adversary.
I'd argue that it's marginally less attractive. They still have the opportunity to inspect some unencrypted traffic.
And even adversaries contribute useful, secure bandwidth to the Tor Network.
This could also potentially backfire: adversaries can create local (non-tor) policies to throttle encrypted exit traffic, while not throttling unencrypted traffic.
Peter..