I want to create a server for a customer and have that customer finish the configuration for themselves. It was been decided that rather than setting default DNS servers (i.e. something like Google's) that the customer should enter the information by themselves. I assume that the customer is technically competent enough to do this.

If however they forget or neglect to set this up they might spend some time trying to figure out what is wrong and eventually contact support. (In this case, I think that setting a default might have been better.)

Apart from the obvious inability to resolve hosts, what other issues might they face until they have set valid dns servers in resolve.conf?