DF is a "create your own fun" kind of game. If you try to play in an "optimal" way by exploiting the ridiculous trade values of some items and giving your fortress just one entrance with a drawbridge you won't have as much fun indeed. The solution is to simply not do that and set your own rules and goals.

I'd even say that trade and combat being exploitable is a feature, not a bug, since sometimes I simply don't want to bother with those aspects of the game.