I feel that only by taking at least fuel according new (provisional) destination which is open at +/- 1hr gives you the same level of safety. Of course, if original destination opens during flight and you have enough fuel you can land there and save your company some money.
By just having two alternates for closed destination we would have to comit to one of them well in advance with no plan B. That would be same like in cavok day we have to chose between destination and alternate well in advance. Normaly we dont have to, we can choose destination, have some bad luck and still reach alternate
And for your comments that as Captain you can take extra. Yes, on a good day, but how much excatly? We should know exact figure for minimum safe fuel.
As you figured out we are new to JAR and a bit dissapointed that it isnt clear as it should be.