This excerpt provides one explanation as to why RF pentodes usually had a separate suppressor grid pinout:
RCA stressed this feature – separate suppressor grid pinout – for its RF pentodes at least from the 6CB6 onwards. It was said to allow an unbypassed cathode resistor without risk of regeneration, and in turn a partially unbypassed cathode resistor minimized input capacitance variations, and hence detuning, with changing agc bias.
So basis the RCA position, the need for direct earthing of the suppressor grid would appear to be circumstantial, and not necessarily general.
It is interesting that in moving from the EL42 to the essentially identical EL85, Philips also changed from an internally connected to a separately pinned-out suppressor grid. Whereas the EL42 was seen as an AF valve, the EL85 was positioned as an RF valve that could also be used at AF.
Cheers,