As clever as smartphone cases can get, their features are usually set in stone; if you decide you need a battery pack after you bought a wallet case, you might be stuck. Logitech hopes to eliminate that accessory anxiety with its new Case+ add-on for the iPhone 5 and 5s, built in part by the TT Design Labs crew it acquired last year. The modular design starts with a basic protector, but lets you attach components depending on what you need.