The heater core will still dissipate heat into the cabin even if you don’t turn on the blower motor. Maybe it won’t blow out of the vents, but it will still raise the temperature inside.
Also, it depends on your AC. If the new AC unit uses the same vents and box as the heater core is in, well, when you turn on the AC it will blow that hot air too and you will significantly reduce the ability of the AC to cool your car.
A manual valve in the heater hose is really easy to add, you just turn it off when it’s “AC season” and you’re good to go.
