Agree with Over+Out BUT...
Being low on fuel probably means the pilots screwed up. Even if they did everything by the book and made good judgement calls, it's very difficult to admit that you've got it wrong or you're in trouble. It's even harder to ask for help (just ask any alcoholic) And even harder to make that admission in front of however many dozens of others on frequency.
There is no rule as to when to declare an emergency and (admit it) we'd all prefer just to "ask nicely" in stead of having to admit we're in trouble and then go through all the paperwork. After that you've got to live with being "the guy that nearly ran out of fuel and declared a mayday"
If a pilot says he's low on fuel, he probably is (unless he's just trying to jump the queue) and needs to land quickly. It's not the "professional" way of dealing with it, but it may well be the safest.