It's not really clear in DRIVERS-111 - I see that Robert Stam writes:

| All this secret stuff we do behind the user's back (like rerouting some commands to primaries ignoring the 
| read preference) is evil.
| 
| We should just route the command to wherever the user told us to route it and let the server return an error if | it can't execute the command.

And I very much agree with that.

As for the > - there are (and as far as I know) never have been comments in mcon/read_preference.h about it. There is nothing there now. I would suggest to create a new define "MONGO_RP_PRIMARY_SUPPORTED" or something to make it clear?