Hitting is composed of mainly 2 things: hitting your opponent and breaking through its defenses.
With a touch attack, you don't take into account the armor(s) of your opponent, since you're not trying to break through it. Nevertheless, when you're trying to make a regular attack, some of your attacks miss, and some of your attacks hit, but don't break through your opponent's armor.
If you used an energy (flaming, icy, etc.) weapon, and you hit your opponent, but not enough to break through its armor, you don't deal energy damage. That's why i think you shouldn't be able to do touch attacks with your energy weapon.
Maitre D