Well a cyber attack is more than someone trying to steal your bank details!
Let's face it, these attacks will be state sponsored and if they are carried out, the results are likely to be quite serious. Therefore you need to have a deterrent of some sort. Saying that we will respond with nukes will make whomever think twice.
Not that I would wish to see them used, but the nuclear deterrent has been around for some time and it has been effective.