SAW gunners (at least in the USMC) don't usually have a sidearm, even our M240 gunners would usually carry the M240 and have no M9 of their own, and the Platoon Sergeant and/or platoon commander were usually all too happy to let the M240 gunner keep their rifle in the truck and take their M9 to go with the M240. This might vary a bit unit to unit.
Jim, neither person you referenced had a rifle as a primary weapon, I submit they would have been better off with a rifle instead of a pistol in both circumstances, but I suppose we'll never know. We still have grenadiers in the USMC that carry the M32, and they carry an M4, not a sidearm. I actually wonder why they didn't give your brother an M16 to go with his M79, I figured M79 gunners would have a rifle as well as the M79 is much lower profile than the M32 and those guys always have a rifle.
Yeah... a pilot will always have some means of armament if they go down. I've heard of both rifles and pistols being issued, but someone else can better comment on this.
Your point that a handgun is always better than no-handgun assumes "if unarmed a handgun is better than no handgun," which of course is true but is not the case or at least should never be the case in today's military.
Mike