Electric Immunity? OK, Sandstorm Immunity. A STAB that actually hurts Psychic and Ghost types, some of the most common users of Secondary moves. A STAB that hurts Jirachi a lot more than Fighting ever will. A combined type resist that allows to resist most common status users, except Grass, and Waters.
CAP9's main type is Dark so I don't know what you mean by Psychic and Ghosts, especially since most Ghosts have Levitate. Alright, you got Jirachi, but let's see you hit the Scarf'd version as it U-Turns for Super Effective damage. How does it resist the most common status users when Grass is probably the most common user? Dark/Ground will get eaten alive by Breloom and even Celebi, who can take un-switching Pursuits due to it's good bulk and crush with Leaf Storm.
Not to mention, unlike Fighting, both Poision and Ground actually help to acomplish the CAP's aims, not leaving it 100% to the ability is nice, isn't it? Considering no abilty can stop the Speed drop from Paralysis, and the damage from other passive things reliably, at once. Don't say Posion Heal. It dosen't work without a prior turn, and switching in on Status is something CAP9 should be able to do.
Quick Feet stops the speed drop, but does not prevent full paralysis so you're still incorrect on that notion. Even as a Ground type, switching into Jirachi can easily let CAP9 get paralyzed from Body Slam and then Flinch-Haxed to death, so that's not a truly safe switch in either (not that anything but Ghost or Limber or whatever, really is). I won't mention Poison Heal, even though Breloom doesn't have much issue, but, why can't we leave status up to the ability? There's no law against this and there's a lot we can do with the ability to make up for Thunder Wave immunity. Ground doesn't even let CAP9 reliably beat Rotom-A or other Ghosts who commonly carry WoW, so we'll need an ability to do this anyway.
How about you give me something that Fighting has over Ground, except lack of weaknesses, which will actually help aclomplish the CAP's aims?
Fine, you want an advantage? Usability. Yes, it's theorymon but when CAP9 switches in to take a Latias/Starmie Trick, only to be 1/2HKO'd by Surf, or Celebi Leaf Storm then what was the point of even making it Dark type? Ground totally messes up most of the point of Dark, which was considered a more important type by the community. Hell, it's even weak to the common partner of Electric attacks, Ice. I don't see the point at all. Fighting might not fit the "concept" better but it will at least give CAP9 the ability to not roll over and die to what it's supposed to counter. Poison is probably a better choice than Fighting but both are far superior to Ground IMO.