The weakness of that approach is that you make the decision between OB and DB up front, during character creation, and lose the ability to make the decision as part of combat strategy. Melee becomes roll the same thing every round. Having the option to go on the defensive or all-out-attack is strategically interesting.
I do think that the ability to defend (which is parry, block, dodge... "parry" encompasses all those things) is a trainable ability, but no one ever learns to fight without training those things. If you learn offense without defense, you're a lumberjack swinging an axe, not a fighter. The only place it breaks down is when you don't have your weapon....
this is true, but can easily be accounted for by adding "stances" or "maneuvers" to the game.
You want your character to go on the Offensive, taking an aggressive stance in combat, the Player declares this for the round and is able to trade some DB to OB, Call it "Pressing the attack!"
Or He wants to go on the defensive and starts retreating/move away from his attacker!. He can trade a portion of his OB skill to DB. Though I would put both situations at a 2-1 trade. You get less than you sacrifice. By going on the offensive and pressing the attack, you make yourself more vulnerable. I would also apply a Limiter that you can only "transfer" an amount equal to the other skills ranks.
So if a Character had 6 ranks in OB but only 2 in DB he could only transfer 2 ranks worth of OB to Defense and would only gain the benefit of 1 rank ( at a 2-1 ratio).
And, of course, a Character that has not trained in much Defense, Say "Gunthar The Brute" Who is really strong but not so bright, so he relies solely on his brute strength and Hitting the guy Hard and fast to win his battles, will be at a disadvantage cause he doesn't have a very high defense and doesn't have much ability to trade back and forth.