I use it so I can have building material. The Save Editor is limited to only what you current have in your Inventory when your using it. With the Duplication Glitch you can get more items for building while your in-game and don't need to continuously Quit the game to mod your Inventory. So, in short it's quite easier.

Well im not going to go spend hours of my time looking for diamonds so might aswell dupe

Well, I use it to help me get diamonds so i don't have to spend hours on end trying to find them. building material to build my mansion and city. Lastely for wool so i can make pixel art

I dnt get why they spent time patching the dupe glitch when once they bring creative mode then the duping would stop,for most cases...

without it it would take TOO LONG to make decent pixel art because sheep dont just grow will instantly it cant take up to 10 mins just 3 more block of wool
another reason for duping is diamonds not many people enjoy diamond searching for hours just be able to make a diamond armor set with all diamond tools

also, its not that people who duplicate items *cant* get the objects themselves its less tedious. :|

as for inv editing i have no idea how that is useful

It's your opinion but I personally don't care, it doesn't affect your gaming.

i dupe cause its like creative mode

Exploiting a glitch in my opinion defeats the purpose of Minecraft.

Duplication is needed because the lack of creative mode