I've been training solo for 20 years, so I may not be privy to new practices, but I can't imagine a scenario when you'd spot a deadlift. There's no danger if the lifter drops the weight. The only possible injury is a blown knee or spinal issue, in which case dropping the bar is your out.
Because of the amount of weight, I'd think failure is failure. I would never want to force reps on deads. That's begging for a bad back injury.