We clearly have different definitions of 'intuitive'.
You've clearly got this all completely backwards. I don't want to have to figure out match problems while I'm trying to roleplay. Therefore, I don't want to deal with bell curves at all.
You've got it completely wrong. As should have been apparant, I'm arguing from the perspective of the DM. I'm not suggesting that PC's need the exact odds of success. I've famously argued on several occassions that the PC's don't even need to know what the underlying rules are. I'm suggesting that at all times DM's need to have good ideas of how difficult (or not difficult) the hurdle they are demanding to be jumped is. The fact that the example that you provided involving the +2 arrow is in fact wrong only reinforces my opinion that my definition of intuitive is quite different than yours. The absolute increase in expected damage is not nearly as important as the relative increase in expected damage, and the relative increase in expected damage is higher in both cases when the task is harder.
For example, if I need a 17 to hit with a weapon doing d6 damage, my expective damage per arrow under d20 is 0.7, My expected damage with a +2 to hit weapon weapon is 1.05 Since this is 50% greater than my normal expected damage, and hense cuts the time require to defeat the foe by 2/3rds this is a time to use the +2 ammunition. In contrast, if I need a 6 to hit, the expected damage with normal ammunition is 2.62 per arrow. If I use the +2 ammunition, the expected damage is 2.98 per arrow - which is less than a 15% increase in effectiveness. Hense, this won't significantly speed up the encounter, and now is not a good time to use the +2 ammunition (or waste actions trying to gain a tactical advantage). So, your intuition about the numbers was in fact wrong. In D20 its absolutely important to hold your good stuff for when its needed.
So, tell me what the equivalent situation is in a 3d6 system. How much faster can you expect the combat to go when you need a 17 to hit and switch to a +2 to hit weapon? How much faster can you expect to the combat to go when you need a 6 to hit and switch to the +2 to hit ammunition?