I had assumed the purpose of joining the hands in the rhino guard was to make the guard stronger and less likely to be knocked aside.
I’ve been taught similar concepts, though with important differences, by a 4th degree BJJ black belt with EXTENSIVE experience in kickboxing, Silat and other striking arts. IIRC the Gracie combatives include some very similar concepts.
He too, developed a strategy to allow a grappler to close with a puncher without minimum risk (which is not to say NO risk - there is no such option).
However, his guard does not extend out from the body like the rhino guard does. His two main guards are the “shell”, where you basically have both hands on the hairline at the front of the head (open hand if no gloves, fists if using gloves), elbows down and with a tight structure, very similar to Rodney King’s “crazy monkey” defence. This structure is very strong, and to my mind superior to the rhino guard as it gives no chance for the guy to knock your hands back into your own head, pull them down or attack them with grappling moves. Standing, it is easy to move the elbows slightly to block any head shot, and even some body shots. You basically just charge at the guy until you get both forearms on his chest and then are in clinch range with your head on the guy’s chest. The guard easily then converts to over/underhooks, body locks, arm and neck ties, etc.
The structure works well for a closed guard on your back in a punching situation - pull him in with your legs, keeping the shell on to avoid punches, until his chest is on your forearms, from where you can get an overhook and neck tie, move to rubber guard, head/arm choke, etc. I wouldn’t want to use any guard that took my hands away from my head if I was getting punched underneath on the ground.
It is even arguably the best of a number of bad options if you are mounted and being punched. Just weather the storm until you can buck him forward and hopefully grab an arm to bridge and roll.
The other option is a thing called the visor, where you wrap both arms around your head, a bit like as if they were folded, and you peek out the slit between the forearms. Same thing, run at the guy until you make contact with the arms and go into your clinching game.
Here’s an example (I would call this the visor, rather than the shell): https://www.youtube.com/watch?v=mftSpRTuCJg