...quite worrying when you think about the margins these operators are working on.
Which margin? If it's profitability and they owe you money, then yes I'd worry. Not all airlines are brilliant at making a profit. But I wouldn't worry too much about the safety aspect. You start running out of fuel as soon as you start burning it at your point of departure. But more importantly, you don't continue unless you have a way out - ie. generally a destination and/or alternate with appropriate weather. If that looks in doubt, you stop off on the way an fill 'er up.
As for money, my little bit of the our operation does 100,000 flights per year. 20 minutes holding per flight would cost something like £2M. So we can do a few diversions before we are in a loss. And don't forget, if we think we will have to hold before we will put on extra fuel. And to put things into perspective, I've diverted about dozen or so times in 20 years (500 flights per year) and on each and every diversion I've had plenty of fuel. But I've regularly landed at my destination with less than alternate and final reserve fuel!