M. Mouse, lamina, others
My point has nothing to do with JAR-OPS fuel policy, just airmanship.
Let's assume you are unfortunate (careless...?) enough to arrive at OCK or wherever, and the only way you can hold for 20 mins is to use your diversion fuel.
20 mins later, you have only one option: land at LHR. If LHR closes for some reason, you're toast. If the EATs suddenly extend by 20 mins, you're toast. if somebody screws up in front of you on approach, you are (probably) toast.
If you divert, you get to your divert and it closes, you are still toast. If you get halfway there and it shuts, you can still get back. See the point? Diverting does give you slightly more in the way of options (but only slightly). Both situations are undesirable, but one gives you better odds than the other.
The point is, arriving with so little fuel that you have to use your divert fuel at all for holding is stupid. You either carry enough fuel in the first place, or you tech-stop. To do anything else lacks sense.
Or maybe somebody can explain to me how stretching fuel to absolute minimums is sensible...???