The reason extra fuel is not encouraged is the cost involved. On a long range to LHR, you will burn one third carrying it. So 6 tons on top at SIN or BKK will be 4 tons at LHR, quite possibly leading to offloading pax or cargo to cget 6 tons on board. Any wonder policy is not to carry extra fuel unless there is a good reason? Contingency is not much, and can easily be used up (and more) if height restricted.
I will either divert very early or enter the hold with the firm intention of committing to destination. I see no reason to then easily divert from a destination with 2 runways to a single runway alternate where your problems can be worse- my experiences of diverting have always been total chaos- even from LHR to LGW.
I would have more patience with someone who felt he had been inadvertently misled about an EAT. Just last summer I was caught out and had to declare 'Pan' on a go-aroundn with already low fuel. It is a horrible feeling. Diverting is not the panacea people make out.