When a team is preparing for a championship football game, do they practice by telling the practice squad to wear blindfolds and earplugs; then celebrate whipping the blindfolded teams ass? No. They practice by playing against a well prepared squad so their mistakes can be corrected before the big game.
Does a heavyweight boxer, preparing for the championship bout, choose a featherweight as a sparring partner?
Does a professional tennis player climb the ladder by playing only opponents they can clean the court with?
Does anyone get better at anything, by pitting their "A" game against a "C" player.
If your goal is to elect better Democrats, do you throw anyone who dissents, brings up weak points or shows chinks in the armor of your choice for your party? Knowing the other party will surely use the same points against your candidate?
If you have half a brain, you use the tools you have, to sharpen your candidate and prepare them for unfriendly fire.
Because, in the real world, you would be fired for incompetence or ridiculed if you were the owner.
Especially if you built your team on the backs and through the donations of others.