WhatsApp does require a substantial amount of device access. It needs access to SMS and contacts, of course, and asks permission to create, use and delete contacts and set passwords. It tracks the user's approximate network location and precise GPS coordinates. It gets to modify or delete data on USB storage and permission to use all available network connections, to read the phone status and contact card and to make calls. It needs to modify system settings, install and uninstall shortcuts, take pictures and videos, record audio and test access to protected storage. It runs at startup and knows the status of other running apps. It might have an effect on battery life; it can prevent the device from sleeping, control vibration and toggle sync on and off and read sync settings and stats. The upshot is never having to log in; it's always on and always connected, just like SMS. When Wi-Fi is available, it automatically switches away from the carrier network.