Well, every case is different. I had same situations but employees were made happy (money and role are still good motivators), some left sooner, some later but they did the job well in the meantime. In one case I said "go for it" because the position was much better (an employee was telling me about every stage of the recrutation from the beginning, I lost great guy but it was indeed better for him). Generally, I agree to be honest, every single time when I was leaving I wasn't negotiating for a counteroffer. But if somebody wants to stay and have better terms, it's a valid strategy.