Now that you have revealed that the context is an advertisement for a
heart valve (the best according to the ad!), it's perfectly clear
(except for the labels doctor and patient).
The heart valve manufacturer is saying to the prospective customer, a doctor:
You would never tell a patient that you were going to implant the
second best heart valve into his body, so why would you ever actually
do it, that is, why would you implant the second best heart valve into
his body. Buy our (first best) heart valve and save yourself the
embarrassment of using a second best product.
With that in mind:
Question:
1. 'd = would / could / should? would (It can never be could or should, by the way.)
2. You'd never say it = Really ? No. It doesn't mean really.
It means you would never say what the doctor said in the first
line. You would never tell a patient that you were going to use
an inferior product for his operation.
3. Why do it? What does it stand for? Does it refer to the implantation operation? Yes. Why do it? = Why do the operation (with an inferior product)?
CJ