It really depends on how deadly and cinematic you want the game to be. I had thought about doing this before, but came to the conclusion that it really wasn't necessary. Here's why-
1. BABs are universally lower than in other d20 games (assumes multiclassing with non max-BAB classes).
2. Armor adds to AC (some people seem to forget that.)
3. Taking cover adds up to +10 (I think) to AC with 9/10s cover. However, since the rules for cover aren't in the THB (as you are supposed to take the rules from a WOTC book), I have a feeling many people don't know that either.
Taking those three things into consideration, I feel that adding a further class Defense bonus is overkill and will tend to drastically reduce the number of hits in a combat.