This is an interesting discussion. Please forgive lengthy post that follows....
I think there would be very few people who would genuinely argue in favour of having animals in enclosures which made seeing them a 100% certainty, as in some toy zoo. To suggest that people are arguing for such a thing is to practise the art of reductio ad absurdum, I think.
However, what many - myself certainly included - would argue would be that the primary reason for going to a zoo is to see animals. Thus, a visit to London (and, to be fair, to many similar zoos) can be a frustrating experience, for a number of reasons:
1. Enclosures are sometimes set up to make seeing the denizens within very difficult - either through bad design (such as with the anteater enclosure) or by (possibly rightly) really skewing the balance in favour of the animals' needs, rather than the visitors' (such as in the nocturnal house, where the darkness can be very dark!).
2. The extraneous tat described above - which seems to have reached its apotheosis in Land of the Lions - can take precedence over the living animals. David Hancocks once wrote that the actual animal was sometimes only secondary within a zoo exhibit; I fear we are reaching the point where his words are being validated. Some will like this. As is mentioned above, the railway theming is, so far, popular with some visitors. But let us not pretend that it is anything other than gimmicky window-dressing.
3. For a zoo which seems, to a large extent, to be run by its marketing department, there is a curious disdain for visitors sometimes evident at London. Or rather, the visitor who wants to see meerkats, ASCOs and ring-tailed lemurs, and have a suggestion of a lion and a tiger (with entertaining theming....) is served very well. But for someone who wants more than this... back off! If there was an off-show tree kangaroo at any one of twenty or more European zoos, I would know that I would be able to find someone who would be delighted to show it off to an interested visitor. I wouldn't even bother asking at London...
4. ....and connected to this, I just don't think that there are sufficient people at London (or many UK zoos) who are interested in keeping and showing interesting animals for the sake of keeping and showing interesting animals. When the RSCC closed last year, few indeed were the British zoos that were the slightest bit interested in taking on an echidna, a fanaloka, a tarsier - because there just isn't that fascination with biodiversity as a wonder in its own right.
5. Any zoo visitor knows that they cannot guarantee which beasts will be visible, active, doing interesting things, on any given visit. Yesterday, the binturongs were snoozing. Today, the kusimanse are not to be seen. Tomorrow, the striped hyaena are invisible. But, if there is a large collection, then something will be active, something will be showing itself. If all of your animal eggs are in one basket - as with London's tigers - then, if the tigers are inactive (as they often are) then you are simply left with an empty-looking tiger exhibit.
This last point seems, to me, to be the crucial one. If the tiger thing had, as part of its general area, an aviary of south-east Asian birds, some rodent displays, a binturong or two, maybe those who weren't lucky enough to see the tigers in action might see the other things. To be fair, the gorilla and lion areas do have other species, which is good (although in each case it has been done rather crack-handedly).
London Zoo is doing well, with visitor numbers as high as for many years. But they cannot afford complacency. It is remarkable to see that Colchester Zoo now receives more than 900,000 visitors a year - and it is notable that at Colchester one is pretty much guaranteed to see things...
