Waste of time and worry. Suicidal pilots have ample opportunity to brain the other pilot with a crash axe or fire extinguisher - or simply to shove and lock the control column forward on short final. In a decompression the pilot is sitting next to a quick donning O2 mask, any helper would be standing and not nearly as familiar, so would be the most likely to have hypoxia problems. In short, an FA on the flight deck isn't worth the effort.