Play the role with the character that has that ability then. Do you think every bio should be able to use the hair jutsu that Jiraiya used aswell? Because same idea, and i'm against that as i think abilities such as that should be unique to the bios. Same as i'm not a fan of Nagato anymore, i dropped him, should we make a rule so any bio can have the rinnegan just because i like the ability but not the bio? No we shouldn't. Or it ends up being how it was like 6-7 years ago when people were running around custom ems bios with ice. Small stepping stones. You allow custom Crystal bios, you get all the people complaining that they can't have custom EMS, or rinnegan or custom wood bios... it was said to be unique so it should stay that way.
There are people who went out of their way to make crystal like CE as they wanted to use that style like Toku's Ruby.
It's the same reason you have specific abilities to other bios. In this case Guren was said to be unique by Orochimaru, the same as Gaara with his sand, hence Gaara of the sand. There are several abilities that would be pointless to give to every bio, for example, Samehada, if you let any bio have it it doesn't give a uniqueness to it. It means that if you really want crystal why not make a Guren bio, if you really want the 4 tailed beast go for Roshi, if you really want Samehada go for kisame or his previous master. I don't see why it would make a difference if you like the ability so much.
I'm not talking about giving any ability to any bio. I'm talking about earning a bio and giving it the appearance, personality, and history of a non-Naruto character because you prefer to RP as them, but you want the abilities of the other. Same restrictions as before. For example, Nagato. You get the permissions, pass the test, same as the normal process for Nagato, and if you prefer, you "skin" it, carrying the same restrictions and rules, no abilities a Nagato bio normally can't have. No one is saying let Kakashi be able to have Rinnegan.
The argument that "Only certain blablabla have certain blablabla in the manga cuz they're special" is a super weak argument. In that case, lemme catch a Kakashi bio using Rasenshuriken or a Neji bio using Chidori or a Sasuke bio using Water Dragon. Can I report them and say they're using abilities unique to other characters and therefore shouldn't be used by those bios?
In this case, you'd make a Guren bio, and then you can skin it to be Aokiji. Or have a Roshi bio and skin it to be Akainu. Custom bios like that can actually work.