I own a couple of "pure & obvious" defensive blades, both folder (Spyderco Civilian, Gerber Folding A-F, Al Mar Jumbo Quicksilver) and fixed (Gerber MkII, Cold Steel Urban Skinner). I never carry any of these. I normally carry either an Emerson PSARK or a Spyderco Chinook. Both are suitable for heavy-duty cutting/rescue tasks. They are not normally used for "normal" day-to-day office chores (opening envelopes & packages, cutting string, etc.), as I carry either a small 3-blde stockman or a Case Russlock (less intimidating to co-workers).
Should I ever be forced to employ the Emerson or Spyderco in my own defense, it at least doesn't scream "dirk/ripping claw/push dagger." It may not make a difference in court; that's highly situational. However, it's one more base covered, and these folders are, for all practical purposes, as effective defensively as a purpose-built defensive knife. It's the determination and skill level of the user that will make the difference; not the exact blade length or configuration.