Mobile devices such as iPhones and tablets have the ability to hot swap between wifi networks that they have previously been granted access to. Desktop systems generally need to be manually switched between networks if the primary router fails.
There are security issues inherent in not being aware at all times which network you are actually currently connected to. A strong unencrypted connection might be made to a router that was outside your home or office allowing third party access to your communications. This is a distinct possibilty in multiple occupancy buildings, where wired connections are always to be preferred in the interest of maintaining data security.