The whistle blower is not an accuser. The WB became aware of something that didn't seem right. Following proper protocol, the WB reported what he/she had learned to the appropriate authority within his agency. That is the full and complete role the WB played. At that point it was the responsibility of those he reported to, to investigate the situation and determine if there was any merit to the claims. This was the case.
It is as if you're saying a person was told there was a hit-and-run auto accident and they called the police who investigated and found there was indeed a hit-and-run accident, now the person who called it in must appear in court to testify. Why? What probative value is there?
The GOP want to distract us from the facts, the real issues at hand. There is nothing the WB can tell us that will bring anyone closer to the truth. They simply want to attack this individual for appearances, nothing more. And it is driving Trump crazy knowing he can't attack if he doesn't know who it is.