Thank you for your comment and feedback :)
I do agree in terms of each app having its own API nodes for both stability and performance's sake. However, we have to consider the fact that it is not easy to maintain a global node network. Especially for apps. Currently, I believe all but one of the public nodes available are hosted in Europe. Setting up nodes in different continents and making them work together in an efficient manner is no easy feat (and is definitely not cheap).
Not to mention that it is important to give people more options to choose from. What happens if hive.blog API is down? What happens if ecency's API is down? Considering @blocktrades' website was hosted on OVH and the datacenter, literally, burnt down a couple of months ago... what if that was the DC API node was stored in.
If I write further, I'll probably go off-topic, but in summary, while apps can and should have their own nodes (not just social sites, all important apps should IMO, but it is important to note that not all apps are generating revenue or have DHF proposals to pay for global node infrastructure for itself) it is also important to offer third-party nodes for the sake of decentralization. A different node operator, a different server provider... a different continent altogether.