Lots of confusion about experience in this thread. Experience does:
* Add to melee attack stat.
* Add to melee defence rating from all directions (defence skill).
* Add to morale.

Experience does NOT:
* Add to missile attack stat. The stat page for the unit shows an increase, but it doesn't actually make any difference.

Try a battle between a slinger unit with triple weapon upgrade and no experience against a phalanx unit, and tally the amount of dead by the time the slingers start to skirmish. Then try the same battle but give the slinger 9 chevrons but no weapon upgrade, and finally the same battle with no upgrade at all on the slinger. You'll see that the weapon upgrade makes them massacre the enemy while the chevrons make little or no difference from the base unit.