There is a very interesting incident report (unfortunately in French only) at
http://www.bea-fr.org .
For those who don't understand French, I'll try to explain quickly:
on 28 August 1999 a french 737, F-GJNF, landed at Paris LFPG with only 800 kg of fuel left.
The Paris area forecast was all CAVOK, but suddenly fog arose, and CATIII went in progress with considerable dalays, of course.
The airport was not closed, but suddenly capable of taking only a fraction of the normal flow per hour.
The captain, instead of diverting to Orly where the weather was good, insisted on waiting in the stack, with no idea of the expected delay, using his alternate fuel as extra holding fuel.
He had to eventually declare MAYDAY on final.
Very bad decision and gambling, obviously.
But the interesting point is that, perhaps surprisingly, he didn't break any legal rule, as noted in the report.
I have to admit I can't find any reference to this aspect in my company manuals.
I once used part of my alternate fuel (thus committed me to destination) in Rome, because of heavy traffic, but the weather was excellent, LIRF has got 3 runways, etc etc..., and I had a precise Expected Approach Time from ATC.
It seems we all agree, substantially, on the conduct to take, but it's interesting to note the legal requirement is quite permissive.