Kids:
Three issues here:
1. Safety of Flight
2. Maintenance Support
3. Efficient Performance in the Cockpit
Not necessarily in that order, but all apply whether you are pilot or NFO, although perhaps they apply more in the TACAIR community than others.
1. If you drop your pen there's a remote possibility it will get stuck somewhere and jam throttles or flight controls. You may or may not be able to muscle your way through it. If it's a plastic government 200nm black Skillcraft pen, it will probably break with sufficient force applied. Perhaps not if you fly with some sporty executive "Cross" pen or wooden Mont Blanc look alike (like some senior guys I knew).
2. If you drop your pen and can't find it, you have an obligation to down the aircraft after the flight for FOD in the cockpit. Ideally you can find it as you unstrap and climb out. If not, the troops will have to look for it. I guarantee that when the jet traps, it will not be anywhere where you thought it would be, but will get slammed forward and slip through the firewall or through the floorboards. In the quick turnaround of carrier cyclic ops, that may mean losing a sortie while maintenance searches for it. The heat vector would immediately be on you. You don't want that.
3. You drop your pen in the Intruder or Prowler, you ask your bud sitting next to you to use his spare (if you forgot yours). Not sure how the pilot tosses his spare back to the RIO / WSO. And, in my experience, the pilot never carried any "extra" anything - as A-4s said, that was what the Bombardier / Navigator / Co-pilot / Secretary / Snack Mom / Psychiatrist / Cheerleader in the right seat was for

. So, you either had to ask for the pilot to unload the jet, and hope the pen floated back up to where you could get it, unstrap and root around for it in flight, or use your spare from your sleeve pocket. But, Murphy's law guarantees that the time you need the spare is the time your pilot borrowed it to sign for the jet in maintenance control and left it on the counter. Then you're stuck having to rely on your memory to get you through things like clearances, frequency changes, marshall instructions, debrief items, etc, etc.
Tie it on your kneeboard and none of those things will happen. I promise.