Since I became total convert to the Cadet (after 45 years of carrying other SAKs), it's the nail file for me. My fingernails get dirty a lot, so I use it for its intended purpose, but it's just a great all around probe thingy... getting bits of nuts out of shells, digging wax out of candle holders, etc. And -- like the corkscrew -- it's good for loosening and dressing knots. And, it's pretty good on small Phillips head screws. And, it's good for cutting or scoring tape on packages, and opening envelopes.
I guess the main thing I use it for is as an excuse... When I'm sitting around, I constantly get out my pocket knife to fiddle with and fondle. With the nail file, I have an excuse. Hey, nothing to see here, I'm just cleaning my nails.