Also why can't I have a full sword conversion so I can go full on ultra Marine on these raider heretics.
The Serrated or Stealth Blade attachments to the Combat Knife make it pretty much identical to the Trench Knife, sans the knuckle spikes on the grip.
As for the Ripper, one loading screen describes it as being used as a close-combat weapon for American troops. It wasn't meant as a stealth weapon, it was meant as another brutal melee weapon like the Super Sledge... and American Troops were probably using it in Power Armor, too. In Fallout 3 you'd have unarmored Enclave officers using it, though.
Would be nice to make it a full 2-handed chainsaw. My Doomgal character would certainly appreciate that.