Damage breakdowns only really work if you're analysing a pve environment as the abilities people use in pvp vary massively.
If you go to simcraft you can see that there are breakdowns of classes abilities, for example;
Assassination rogue
http://simulationcraft.org/505/Rogue...ion_Dummy.html
If you have an understanding of each ability, look at the damage sources pie chart.
"Physical" damage, comes from the following; Dispatch, Melee Main Hand, Melee Offhand, Rupture, Mutilate Main Hand and Mutilate Offhand.
(Bear in mind it'll only affect Rupture if you apply Enfeeblement before the rogue applies the bleed)
If you relate to the pie chart, you'll see that if applying Enfeeblement to an assassination rogue, you'll reduce the damage you recieve by approx 5% (if that). As the rest of their damage comes from poisons.
Classes/Specs that use entirely Melee damage such as Warriors or Feral Druids will receive the complete -20% damage reduction, but as stated before, its effectiveness varies from class to class.