It depends on how the person is pushing themselves. I work people through phases in order to prepare them for a different intensisty level at each phase. If they exceed the range i give them for rep counts then weight is added. Everyone is informed on the risks of adding weight and forced reps and working to extreme fatigue. If a client cant get enough or adhere then i give them what they want to a degree. IF they want fatigue then i take it out of their lungs with intervals between sets. Pushing the lactic threshold in that manner will slow anyone down.
All depends on the situation.