A good knife is probably the most useful tool any person could carry on them; and I usually have a few on me. Aside from my main knife, a Buck 450RD Protégé, and the Victorinox SwissCard Lite in my wallet, I wanted a nice backup knife to put on my keychain. You never know when it might come in handy.
The character "Soap" said in the world's greatest movie, Lock, Stock and Two Smoking Barrels: "I think knives are a good idea. Big, Fuck-off shiny ones; Ones that look like they could skin a crocodile." While I totally agree with Mr. Soap there, the weight might be an issue if I put one of those on my keychain.
My new Victorinox Classic SD has everything but a weight issue. At 22 grams it's hardly noticeable, yet this little baby is packed with functionality. It has a nice sharp blade, a little nail file which doubles as a screwdriver, a pair of scissors, a toothpick and a pair of tweezers. All this goodness, stuffed in a tiny pocket knife which is about the size of a AA battery.
The Victorinox Classic SD is a perfect companion for the little Fenix E05 flashlight that also resides on my keychain. They're both black, they weigh about the same, and they are more or less the same size...
But above all else... they just look absolutely adorable together I think anyone should have them on their keychain.