M.Mouse
I cannot as you request explain my statement statistically, however perhaps I can make it a little clearer what I am trying to say.
Arriving at your diversion having used your diversion fuel does not equate to the same thing as using your diversion fuel as effectively contingency fuel. My reasoning is that when you arrive at your diversion this is a fairly unusual event, and therefore happens infrequently, so the chances of your diversion being closed having diverted are very small. Committing to land at Lhr gives you the same chance of it being closed (i.e. very remote) but because some people do this on a very regular basis the chance of someone getting caught out is very much higher.
In 20 years of operating to Lhr I have only seen it happen twice, but if it even only happens every 10 years, and many aircraft are committed someone will sooner or later get caught out, and this is unacceptable.
Good points about the tech. problem situation and the impact of a low fuel state.