Even without Silences being tossed about, Silent Spell is still very powerful to tandem with a form of Invisibility, because it lets you remain silent and unseen even during casting. With Greater Invisibility, you won't betray your presence with verbal components to make a Listen check to find your general area, so they don't even have a square to attack at 50% miss chance.

This is also how you prevent the +5 bonus on Charms in a Hostile Situation. You move in while invisible, then Silent Charm Monster. Since he wasn't aware of your presence, he's not recognizing you as being hostile. Sure, you show up, but by that time, he's already your best friend, and who wouldn't want their best friend to show up?