Lasiorhinus has it right - they're ancient code from the dit-dit-dah days that aren't much used. Though people still say QNH and sometimes QFE because we all love a little tradition
Personally, I look to find uses for:
- QRM Are you being interfered with?
- QRS Shall ... I ... talk ... more ... slowly?
- QRR Is your auto-pilot operational?
- QRZ What was that callsign again?
- QSI Would you just shut-up!