It doesn't. At least not in general. Introducing defects and/or raising the temperature will always affect the propeties of the superfluid (coherence length etc). In some cases this also leads to a finite resistance due to e.g. phase slips.
This is why we use various limits, e..g the "dirty limit" etc when dealing with real materials.
How much of the Cooper Pair problem have you understood? Maybe people can answer you starting from there. For example, in Chap. 34 of Ashcroft and Mermin, 1st ed., they describe the derivation of the bound state. Have you worked through this? Is this what you didn't realize the significance of or didn't understand why it is important?
You need to be MORE specific in your question than this to get any reasonable answer.