If it was something like that, then yes I would agree with you.
OK, cool, we agree that there should be some sort of criteria to assess whether the witness is relevant and appropriate and whether the questions are relevant and appropriate.
But it's not something like that. By Trump mentioning the Bidens is what got him in this mess to begin with. So we need to know whether or not Trump had good reason for wanting the Bidens investigated.
Perhaps.
But the only way to know whether Trump had good reason, would be to ask Trump to present his evidence of wrong doing by the Bidens.
In reality, if Trump had evidence of wrong doing by the Bidens with regards to Ukraine law then Trump ought to have had his Justice department hand that evidence over to Ukraine, rather than simply demanding a public announcement of an investigation.
How are we going to know that for certain unless the Bidens testify under oath at this trial?
Let's consider the issue of the FISA warrant against Carter Page.
The Judge demands the investigators to bring in evidence as to why Carter Page should be surveiled. The obligation is upon the investigators to provide their evidence. They don't demand Carter Page come in and testify under oath to the Judge whether he is guilty of anything.
You have to have sufficient evidence of wrong doing first, before you can force the defendant to have to defend themselves.
If you really want the Bidens investigated, and if there really is evidence of wrong doing, then wouldn't that be sufficient to open a case for a specific investigation into them? But this isn't part of the impeachment. It would be a seperate case. You don't have to drag this into the impeachment in order to investigate the Bidens. Trump is best buddies with Barr. Surely Barr can open an investigation via the Justice Department into the Bidens, if there is sufficient evidence.