...Well no. Say you have boots that give 10 stamina. Destroy those boots wiht disenchanting. You now have +Stamina enchant permanently. So lets say you get a grand soul gem and boots. You can use it to give your boots +30 stamina. And you do not lose the learned enchant: you can do it for every piece of armor you come across.
Now lets say you invested skill points in Enchant. This can increase the effectiveness even more.
So to sum up: You destroy an item once at any level, you get the base effect permanently in your enchant list, which can be applied with greater effect than the item you got them on bonuses based on soul gem and enchant perks.