Decoy is nice, but it has a few problems...

1: It requires TP to use
2: Its not a 100% protection...its more like FFXI blink than utsusemi.
3: It has a pretty long recast.
4: It will only hold 1~2 ranged attacks or nukes.

So, in short, its a good action for when you passing thru aggressive monsters, where you won't really be in melee range and won't take many attacks (like when going to Mor Dhona from Black Shroud).