There is no need to stigmatize and harass just because someone doesn't get advanced formula. I am certain you don't either.
And no, it doesn't help. Just writing that number in parenthes has zero meaning.
In your original question you asked:
Why is this? What is the difference between
1) Dealing 5000 damage + 15%
2) Taking 5000 damage - 15%
So I did the math to work backwards to help illustrate the different effects of + damage and - resist.
The number in parens represents your total % skill damage. For example if you have 100% increased lightning damage, that means you will be dealing double damage. That translates to:
1322 + 1322(100 / 100) = 1322 + 1322(1) = 2644
With lvl 20 lightning mastery you have +278% skill damage which equates to:
1322 + 1322(278 / 100) = 1322 + 1322(2.78) = 5000
D2:R