Rules as written, combat defense is not a test, it is a 'derived statistic', as such, wounds and injuries do not reduce them.
The same is true for flaws, a person with agility, awareness and athletics 3 in each would have a combat defence of 9, even if he had a flaw in any of those abilities.
I see what you say with wounds should make a person easier to hit, however, wounds are already pretty crippling in a fight as it is, having it affect combat defense would just make a fight the first to receive a wound. Though it does affect agility and fighting passive tests so it does make it easier to manouvr and knock down your opponent or distract them.
So as to answer your question, wounds and injuries don't affect combat defense for me, however, they do affect passive aimal handling tests in jousts, an inury gives a minus 1 and gives a penalty of 4 (i.e. individual has animal handling 4, so passive test is 4 x 4 = 16. They take a wound, they are at -1 for their stat, so new score is (4-1) x 4 = 12...or 3 x 4 = 12)