Sure, to the visitor, it adds something to the zoo and is pleasing enough. But not only do the bears lose the privacy of den areas being out of public view, but the second outside area to separate the bears appears far, far worse than the option they had at london. As far as I am aware, Lanka, the mother, is usually shut away, so I suspect the youngest female is on contraceptives (this regime could have changed as the offspring both matured). If all the second enclosure entails is a concrete yard, with house access, then I think this is a quantitatively worse option than the facility at London.
I agree that it may not have been that expensive, but when you look at the capital expenditure at the parks, to build a second major carnivore enclosure (and not market it) in the same year as opening a new rhino area seems rather out of keeping with the normal pattern or speed of developments at either zoo. If ZSL are suddenly able to build new enclosures then that is a departure from their current pattern, and you would think possibly there are a few more pressing enclosure improvements that could come before this.
The whole situation looks like another decision by London to offload some unwanted animals to Whipsnade, possibly the staff felt that the bears should be retired there rather than shipped off to the SSP. I get that the mappin terraces are a little messy for such a complex animal, I just feel that it is a shame to just present them in a nice grassy field when actually, if they cannot all live together, there will always be at least one bear in a worse environment than in London. Maybe one of the offspring will eventually leave for the SSP? There is some amazing woodland at Whipsnade, it would have been great to see these animals rooting around under mature trees.