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Are CTS & RTS frames only necessary in hidden node or exposed node scenarios?  In real-world, I see them where I don't expect to have a hidden node.

This is only a partial answer to your question: 

Some devices had RTS enabled at all times.   Often older devices had a low RTS value set, in their factory default settings,  if they were expected to be used in especially noisy environments.  

You may find some radios that don't have the best sensitivity to begin with have low RTS values, to meet a (usually range) requirement.

If all you see are CTS's, and no RTS, then it is most likely a newer device.     I think /ac might have its own rules too.

Thanks for the response, Howard. 

I've noticed about 1/3 of the traffic is RTS/CTS at times which makes me wonder how that affects the overall throughout since control frames are sent at much slower speeds. I've read a few blogs that state RTS/CTS will help overall throughput in busy environments by reducing retransmissions. I can see that in high density areas where channel reuse is problematic, but I wonder if that's still true when reuse is not an issue.

The concept of setting low RTS values, is an older technique that is usually not even available with modern device drivers.   With good network design, it pretty much proved superfluous.  Manufacturers had more support calls when it was misused, so they removed the ability to modify it.   Newer devices have pretty much replaced RTS with CTS-Self when the need arises.

Removing /b devices from the network will remove the necessity of newer devices having to use protection.

Check the MAC address and look up the OUI, and see if you can figure out what device(s) are sending the RTS's.   You may just have a single old device in the area that you can convince the owner, or management, to replace.

See the forum posts for the "RTS/CTS in 802.11g network with no 802.11b clients"  item that is circulating right now, for further information on your question.