The mongotop --host documentation notes the syntax for connecting to a replica set. Afterwards it states:
You can always connect directly to a single MongoDB instance by specifying the host and port number directly.
I find this information ambiguous. Especially considering this comment in TOOLS-506, I believe that a "single MongoDB instance" includes secondaries in this context (therefore mongotop should be able to connect to them if not using the replica set syntax). However, others believe that mongotop can only connect to the primary of a replica set. Output from the primary is the case when using the replica set syntax but not the behavior I would expect with the "single MongoDB instance" syntax based on my understanding (pending the results of TOOLS-1166).
The documentation should be updated to clarify what a "single MongoDB instance" means as it is leading to different interpretations of how the tool can be used with members (secondaries) of a replica set.