It is important to have both. You don't want all your name servers
sitting in the same rack (plugged into the same 6-socket electric cord),
nor do you want all your name servers sitting on the same subnet. It
doesn't matter how many you have, if they all go away due to the same
problem.$ whois intur.net
Registrant:
Inturnet, Inc. (INTUR-DOM)
811 Alpha Drive Suite 331
Richardson, TX 75081
USADomain Name: INTUR.NET
[...]
Domain servers in listed order:
NS1.INTUR.NET 206.97.151.9
NS2.INTUR.NET 206.97.151.8
NS3.INTUR.NET 206.97.151.14
NS4.INTUR.NET 206.97.151.15--
Sean Donelan, Data Research Associates, Inc, St. Louis, MO
Affiliation given for identification not representation
Sean,
You neglect to note that these are likely topologically diverse
and passed around as host routes.... 
Either that or they are virtual
interfaces on the same box...