Actually, I don't think you'll be ever like this in a combat situation: "OMGstuff! I forgot to clear my mines!" And if you really want to clear your mines while under fire....
/facepalm
But you know the TF2 sticky launcher? The cap for the mines makes it so you can't place more
People can clear quickly and then bait other players into a building where the entrance is mined, and then boom. Otherwise they'd have to shoot them.
Or, set off mines instead of clearing them up? What if you placed them in a area where you'd blow yourself up?
Edit: Maybe the latter won't happen frequently. If so, who would TYPE clearmines? I guess it doesn't hurt to add that in, although people would set them off rather than typing in a time consuming, one second command.