The implementation of new viewing areas for Passage through Asia in Monkey Forest definitely argues against the very heavy (£30) fee to bring a car into the zoo.
That said, I would still recommend it as Whipsnade is huge and managing to return to areas multiple times in a day is all but impossible by foot, yet with a car, staying until late and performing one last 'lap' of the zoo, stopping for any animals you missed earlier in the day, can work great. This is particularly effective for the cheetahs, lions and aardvarks, which from experience tend to be more active later in the day. And the whole Africa zone is much nicer as the sun begins to set and crowds begin to calm down. It is also very useful to allow you to make shows on time (the bird show is a must-see) without having to plan your whole route around it.
It is worth knowing that the zoo's reindeer are not domestic, but wild of the European Forest species, which can only be found at one other UK zoo. One of the penguin enclosures features Northern Rockhoppers kept at just five other European zoos. Another exhibit that is easily missed (it indeed took me many visits to discover it) is 'Owl Wood,' which is in a woodland between the entrance and the Butterfly House. No rare species or notable exhibitry, but necessary for completeness and a lovely setting for some aviaries. And of course, don't miss the gaur.