Deep, deep waters my friend. As a metter of fact, the rubber guard has many techniques similar to other guard systems For instance, Demian Maia's triangle choke is a part of the 10th planet system, but it is also part of other systems as well.
The rubber guard actually seems to work better in mma than grappling, if you watch Aoki and those guys, but in pure grappling Marcelo Garcia's sweep based guard might be the better alternative. However, as you realize, a lot of Marcelo's guard game is not ideal for mma.
I personally like some rubber guard techniques, but I don't have the whole system as a go-to. I grappled first with the Gi and then without and in my oppinion, the gi made me a sloppier grappler; I wasn't as tight as you have to be without the gi. (as of now I train at an MMA gym with strictly No-gi)
A thing though: The rubber guard should not be used against people with good defense. A friend of mine at my gym has trained about the same time as me and is doing the rubber guard ALL the time. I've just laid a little less time in guard defense and guard passing, and his rubber guard has never submitted me as of yet. (And he does know quite a lot of rubber guard so don't give me the whole "it's the person, not the system". When he faces people not familiar with it he taps asses and takes names!).
In the end, it's just another system which I like fairly much and it should not be overlooked.
IronMan could probably put all this much better than me, though. And he prolly have some good points that negate some of mine and adds what I've forgotten
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