Dia III is an awesome spell to have.
Yes anything can be killed with just Dia II, but then by that logic you don't really need any group 2 spells either, you can still kill DL with Slow I and such. What the group 2 is for is to makes it easier and make the group that little bit better, and Dia III does that by further reducing the mob's defence by 5%.
And for the argument that Angon is enough, news flash: they stacks.
XI Wiki


And as i've said 5/5 is not needed for 1F salvage only, with 3/5 you are good and most of the damage will come from aoe dot (silversea for instance). And if you use whm, regen III will be as much efficient/better since it actually cures you.
