The system may get bogged down in the ever increasing numbers of theorems it can prove. In FOPC, John can't prove that he will be able to start his car, because the proof is based on an assumption that may be wrong. In the real world, almost nothing is known for sure. You want to be able to assume stuff and make inferences, and you want to be able to retract these inferences if you find out something to the contrary. But most of the inferences we make every day are not of this type.
Conclusions are only plausibly true.
Given many instances of something, assume that the generalized rule is true. If I had a sister, I would know it. Therefore, I don't have a sister. Conclusions are dependent on assumptions and may be retracted when the assumptions are negated.