The "hearsay" evidence rule is a legal rule only applicable in the courtroom. As with "circumstantial evidence" it can either be worthless or compelling, depending on the circumstances. But there is no reason to discount it if the circumstances make it appear to be reliable.
But in these "hearings," there is nothing that is in doubt. We know what Trump wanted; we know exactly what he said; we know that the career diplomats were collectively clutching at their figurative pearls; we know that the aid was timely given to Ukraine; we know that the threat/extortion/QPQ was never clearly articulated to the Ukrainians, and we know that, despite President Trump's imploring them to investigate the Biden's and make a public announcement of same, THEY NEVER DID THAT, but they still got what was promised.
What the **** does Schiff hope to accomplish by calling a parade of witnesses to re-state what we already know? There is nothing new here. No smoking gun. No revelation. The President wanted to do something improper. He thinks and does a lot of "improper" things. If you don't like it, DON'T VOTE FOR HIM! That is the remedy.