To be fair, Spektrum really is the only manufacturer that has an issue with radio voltage, at least as far as I know. 3.5v is the cutoff for them. At that point, the receiver shuts off and will reboot itself when the voltage rises above 3.5v. It takes about a second for that to happen, so it may cause a crash, or you may not even know it has happened at all. Other manufacturers saw the early problems Spektrum had back when the reset time was 3 seconds and designed receivers that can work with lower voltage. Realistically, Futaba, Hitec, and JR receivers shouldn't experience brownouts with any 4.8v pack that is appropriately sized for the plane it is in. That said, you pay a 1/2 ounce weight penalty for that extra cell and maybe $5, so there isn't much reason not to have it.