The problem is if you had left the HOTAS selection as "activate" and switched to [] but didn't want to jam you would end up with a brief jamming signal before deactivating it. You just flashed the entire RWR world of your presence. And that's if you notice right away that the jammer is active.
Better to have it on standby when switching to [], if you intend to jam right away it's just a quick tap of the HOTAS and you're set.