None of the four lawyer witnesses (who have witnessed nothing) are Trump supporters. Three will do everything they can to impeach Trump. One may be more honest about the Constitution but doesn’t like Trump at all. Aren’t there any professors at Hillsdale College, for example, that support PDJT?
My guess is that the GOP selected him with the hope that he could provide a rational answer to those who consider themselves independent, and haven't made a decision on impeachment yet. Trump supporters don't need to be convinced that this is a national disgrace. If Turley can appeal to some who are undecided, that is a net plus.
I guess I place more emphasis on the law. Whether Turley supports President Trump is irrelevant to me. If he's right on the law, that's what matters, to me.