Why Rhodium?

 Rhodium is a bit different than the other metals. Of course...I know we'll that you gents know that. However what I mean, is that rhodium isn't a very good conductors of electricity. It's actually horrible. Better still...it actually repels it. And if said material repels electrons then we've got generation of electricity. As with a magnet. So what I saying is....rhodium is a catalyst. And it's a magical rule breaking reality altering metal. Do you see?

It's shiny, but most of the time I find it too hard and slick to offer good grips.

I like my contact metals soft and springy. Hard spades for instance are nearly impossible to tighten well long term. Gold over copper however has enough malleability that the speaker terminals bite into them when tightened.

In power connectors they seem to make it a lot easier for plugs to slide in or out.

I love how they look.  Like jewelry, but never had a good experience with them on a practical sense.

I use Oyaide C-004 on IEC and Viborg Silver/Rhodium on Schuko in my system. My observations is that the plugs not only have an effect on the frequency repons (less warm, more neutral) but also on the time domain. The hard metals don't have the "soft" 'decay' of gold and silver and present a tighter sound with more 'attack', punch and control. They suit my taste of music (rock/pop), but I understand that not everyone likes them.

1. Poor conductor
2. Hard which makes for reduced mating contact surface.
3. Only used in audio
4. Uneducated consumer

I am sure there are more reasons.

Current estimates are a leading edge semiconductor plant costs $10-20 Billion (that's 9 zeros). Even a large geometry analog fab is high 100's of millions. I.e. price is really no object. You will not find one rhodium plated connector in that plant.