Most DDOS attacks against a company like yours are actually attacks against a specific customer. If:

a) you had a pool of DNS server names, say 20, all with unrelated hostnames

b) you assigned 2 to each customer, randomly, when they configured a domain to use your servers.

Then, a DDOS attack would impact 10% of your customers instead of 100%. (Assuming other practices, like null routing the target until resolved)