As far as I know they are probably only needed with certain cable types. Think that the RCA plugs may be calculated into the design of a 75 ohm digital cable (or @ least they should be it would seem).
I have experimented with eliminating the binding posts, metal RCA plugs and IEC outlets from some of my gear and the most positive results came from removing such items from the signal path (IEC's were a waste of time - with my gear anyway).
Thing is that removing these items may not please everyone as they may very well prefer the different type of sound I detect with metal RCA connection VS a less massive one.
Also, I have found less of a difference, between the two, when using stranded wire designs. When using simple small gauge solid core wire designs the difference is more dramatic (again, with my gear/ears).
If you have contemporary binding posts (the type with a hole across/through the center of the post and bare ended speaker cable try the following excersize:
Connect one speaker by placing the wires in (not through) the holes and hold the wires stationary when tightening the binding post (so that the wire does not wrap around the post). This will be not be changing the metal mass of the connection, but it will be reducing the contact area that the wire makes with the binding post).
On the second speaker wrap the speaker wire around the binding post a few times (creating a larger contact area). This is the way I see most bare cable attched as we have all been told that a larger contact area is better in the past.
Set back, listen, for a few days to see if one channel sounds different and/or better. If you hear a difference then perhaps try reversing the two types of connection per speaker and see if you come up with the same results. If you have a mono switch you might try it as well.
Anyway, if the contact area alone makes an audible difference then imagine what reducing the metal mass of the connections will offer.