Capping unused rca & xlr connectors???

What is this supposed to accomplish? Probably subjective but is it snake oil or does it provide quantifiable benefits?

It is customary to short any unused RCA inputs on preamps. This prevents pops and clicks when switching your source selector between actual sources that are plugged in. Also prevents unused inputs picking up stray RFI. You can use pre-made shorting plugs or make them yourself. I don't recommend the Cardas caps because they don't actually short the input, but only shield the hot signal socket from RFI.

Unused XLR inputs do not require this treatment since they are balanced and self-cancel any noise.
You said that you don't recommend the Cardas Caps....what
pieces do you like for this purpose?
If you cut the wire off all the cheap ic's you get with dvd players, and vcr's. Then just seal the end cut with a cigarette lighter, this will even allow you can keep out the insects? I'm sure everyone has a closet full of original manufactuer enclosed interconnects that are just taking up space?
Jeff, the Cardas cap doesn't actually short the input. It only shields it. So it's only doing half of what a true shorting plug does. I'm not sure who's making shorting RCAs these days. I always made my own by taking an RCA plug and soldering a wire (inside the barrel) between the pin and the ring, and then screwing the cover back on.
"The tweak shop" has shorting plugs at $6./ pr. that I think accomplish both shielding and shorting.
They are not just ornamental, they can really help reduce noise. My situation is somewhat unique, but here's what happened.

From the Rowland Concentra tape outs there's a single ended Discovery cable going into a pair of EVS Ultimate Attenuators/Cardas RCA to XLR adaptors into an Apogee Rosetta A/D. The output of the Apogee goes via a balanced Discovery cable into an RME Pro96 card via the RME supplied pigtail. The pigtail has both RCA and XLR input/outputs on one end and a D style computer connector on the other. The RCA I/O is unused and I cover them with Cardas caps. The other day while converting vinyl to digital I noticed that there was a large amount of noise on the input signal. After much investigating I realized that one of the Cardas caps had fallen off and once replaced the noise vanished. The residual noise level on the input meters of my recording software dropped from -38dB to -54dB.

As I said my situation is unique, but the Cardas caps proved a real problem solver for me.