With a pacemaker implant and an av node ablation, why would you want to pace the atria but not the ventricles?

You want both: if you've had an AV node ablation, then you wouldn't have conduction from atria (top) to ventricle (bottom). you would want to pace the atria, then a 1/5 of a second later pace the ventricle - to simulate normal conduction.

Pacing: AV node ablation requires ventricular pacing or atrioventricular pacing, atrial only is not appropriate in AV block.