If your business is DNS and you fail at the basic task of distributing it for fault tolerance then what other explanation could there be?
To make this clear. The cost for running n DNS servers for your domain in n datacenters is equal to the cost of having one (possibly rented) pizza-box in each datacenter. You can list any number of nameservers for any domain. Domains like google.com and microsoft.com have up to 5, for example.
Synchronizing the nameservers is a non-issue. For tinydns it's a one-liner (rsync), for bind it's a few lines of axfer configuration. There is no administrative overhead. You don't even have to worry about host failures much because DNS is resilient by design (that's why you can have multiple NS records in first place).
So, in dollar terms, in most datacenters a rented pizza-box starts at around $30/month, often cheaper. Making your DNS 5-way redundant therefore costs roughly $150/month. You don't have to be google to afford that.
And a potential attacker will damn sure not even attempt to take out your 5 DNS locations. He will go directly for your application instead because that one, in most cases, can not be distributed over 5 locations for a measly 150 dollars/month.