How valuable is a MS/Eq Rhydon to everyone?

Ground isn't a really useful type for attacking.
I only use my Rhydon when there's a fire or electr type defender (almost never) or when there's a Tyranitar and I want to save my Vaporeon and my fighter for other defenders.
I guess my Rhydon fights 5% or less of my gym fights.

On the other hand, Golem with RT/SE is a much more useful attacker and is my backup very often.

I have a couple and hardly ever use them for attack. I prefer the stone edge ones for attackers as that is SE against Gyarados and Draggy. EQ doesn't really give you any type advantages over other common defenders except for other Rhydons and there are much better attackers to deal with them.

Basically... no, not because it's really bad, but because it's not relevant:
https://pokemongo.gamepress.gg/themes/gamepressbase/img/type-chart-gamepress.png

Currently there are no Poison, Rock, Steel, Fire or Electric defenders that are relevant, so double ground type STAB attack is not really valuable, because it will only ever deal neutral damage against most gym defenders (at best), and on defense, it's outperformed by MS/SE by a fair margin.

In the current meta, the most useful Rhydon for attacking is a Stone Edge one.

i use them for snorlax and even dragonites, but i would prefer SE than EQ but i would prefer Golem with RT/SE for Rock situations

he is just a tanky not too strong attacker if not going vs water or grass and can spot nice gym places

Personally, my MS/EQ Rhydon 3100CP does very well against Snorlax which is everywhere still. So I do use him quite a bit. However, since the big overhaul in moves he's been surpassed by others like Machamp, which is just a ton of fun to use.

I mostly use mine to take down Snorlax. MS/EQ Rhydon is in an awkward spot because Ground doesn't have a type advantage against many common defenders, and its raw DPS is good but not great. So as an attacker MS/EQ Rhydon doesn't have any stellar matchups against relevant defenders (the best being Snorlax because it resists the normal typed BS and HB) and it doesn't KO quickly enough to be a great generalist in neutral-typed matchups. There is a decent argument for this Rhydon being better suited for defense since it works symbiotically with the common defender Gyarados since they have similar CP and Rhydon helps protect Gyarados’s double-electric weakness. Plus EQ can be tricky to dodge.

I mostly use mine against some snorlaxes and blissey. Rhydon works well at resisting a lot of blissey's attacks. I normally attack gyms in a team with one other person. If I had a good machamp, this might end up replacing rhydon as an attacker unless I'm fighting a jolteon or amphorous.