if I see someone in this position who works for me, I immediately start looking for their replacement.
Once my living expenses went up and a 15% raise would've absolutely kept me with the company if I'd been given it when I asked, because it was the difference between making ends meet and not. I left the company because I had no other choice when I found a higher paying position.
In every other position, it's been cultural. If you can't make your employees happy with money, maybe you should think about your management style, your communication with said employees, and the culture as a whole.
>>> very short term relief from the real reasons
Unless the real reason is that the employee believes that he is being strongly underpaid. I'd define strongly underpaid as a situation, when an employee is certain that he could get (or already got) multiple job offers with 20%+ higher compensation.
Because at the end of the day, vast majority of people work to earn money. Some employers try to make you feel that you are changing the world and what not, trying to say that it's not all about money, but well, it largely is about money. Some jobs are enjoyable where compensation is not extremely important (meaning that an employee would leave only for a significantly better deal), but I'd say that it's a rather rear case.
At the end of the day, it's a free market. We all have a price tag :)