This page:

http://docs.mongodb.org/manual/core/read-preference/

Says:

"In general, do not use primary and primaryPreferred to provide extra capacity."

That doesn't make much sense. Did you mean "secondary and secondaryPreferred"? Additionally, there's no explanation of why the reader shouldn't do whatever it is the note is warning against. The latter problem may be solved in the course of DOCS-2717.

Anyway it's redundant with the later graf, "If read operations account for a large percentage of your application’s traffic, distributing reads to secondary members can improve read throughput." I'd just delete this note.