Why is this a bad idea? This point is
so far afield, I'm not sure what arguement there is to have.
I *think* what you're saying is: "What's the penalty for the fighter, who was engaged in melee, to 5-foot step back and throw at someone else?"
(I'll note here that this is NOT the point you started with. You're sorta a moving target yerself, ain't ya? I'm envisioning you 5-foot stepping away from the arguement melee and throwing at a hastily-constructed straw-man down range. Too bad I didn't ready an action.

)
The penalty you are looking for is this: The fighter has not attacked his melee opponent for a round using his best attacks. That's lost damage, and a lost chance to cut down said opponent. Mainwhile the opponent essentially gets a "freebe" round to cut into your hit points. Combat is often about attrition, and your melee combatant just got a break.
If you think this
isn't a big deal....hey, I'd love to square off against you.