Someone, one day said item destruction upon unenchantment was probably made to prevent power-leveling enchanting by enchanting and unenchanting the same item over and over again.
That's silly, because you already can't disenchant two items that have the same effect. They'd just have to flip that to you can't GAIN EXPERIENCE for disenchanting two items that have the same effect. And anyhow, UN-enchanting would not be DIS-enchanting; it wouldn't gain you any XP or teach you any magical enchantment effects, it would just produce a mundane item of the same sort as the enchanted item you un-enchanted.
Sure, being able to un-enchant something you just enchanted would save you the (small) hassle of having to carry a load of cheap daggers, boots, and hats over to the enchanting table when getting ready to grind up your enchanting skill. But really, that is such a small obstacle to power leveling, I can't see it being THE reason. And anyhow, doing it that way would also mean you don't end up with a huge pile of high value enchanted goods you can sell for cash for more soul gems.
If anything, I'd say the way it works now is probably MORE abuse prone.