Despite very old recommendations, the Iraqi state provider Uruklink.net kept all of its name servers on the same subnet. Although this is recognized as a poor design, many domain name server operators worldwide do the same thing.
nic1.baghdadlink.net. 2D IN A 62.145.94.1 nic2.baghdadlink.net. 2D IN A 62.145.94.2
The way how I see this that there is hardly any incentive to do proper placement of nameservers. The pain inflicted if something goes wrong is minimal unless you are a billion dollar company doing millions of online transactions. And if something goes wrong and you still fly, maybe a very tiny fraction of the population will appreciate that you did your homework. The above applies to many other good networking practises than DNS related ones. It can also be said that maybe the above addresses are carried as /32 inside the destination AS. They might not be on the same subnet. If the number of domains having DNS´s in the same subnet is large, the number of domains dependent on a single AS for their DNS service is even greater. As you all well know, the usual excuse to do poor job is being too busy to do it properly and if failures come every year or two, this might just hold water. Pete