I don't know how this defense bonus works, so I suppose it represents the ability to defend yourself through parrying, just like Dex bonus to AC is the ability to defend yourself through dodging.
I think it shouldn't go exactly with the BAB, but at least a character with Good BAB should have Good or Average DB but not Poor, and a character with Poor BAB should have Average or Poor DB but not Good.
For example:
Fighter - Good BAB, Good DB
Ranger - Good BAB, Good DB
Barbarian - Good BAB, Average DB
Paladin - Good BAB, Average DB
Monk - Average BAB, Good DB
Rogue - Average BAB, Good DB
Bard - Average BAB, Average DB
Cleric - Average BAB, Poor DB
Druid - Average BAB, Poor DB
Sorcerer - Poor BAB, Poor DB
Wizard - Poor BAB, Poor DB
As an example, the Cleric sounds like a character who relies in defense to the armor worn, and he already can wear any armor efficiently, so I don't think he should also get any defensive training. A Rogue instead, since he's not likely to wear armor better than light, tends to train more on defense based on something else.