Another situation where you might want "SMS-activated web location tracking" is the following.
If you use Google's 2-step password verification on your Google account, you have "per device" Google passwords which you can manage from your Google Account.
Let's say your phone or tablet is stolen or lost. You run to your PC and immediately revoke your lost device's Google password, so that nobody can use your Gmail and Market etc. on your device.
BUT THEN, you find out that you also locked yourself out of Avast AntiThief web commands!
According to my experience, the phone will still connect to the web site, send location tracking data etc., BUT web commands will not be pushed to your phone any more. In other words, you won't be able to control your phone from the web, correct me if I am mistaken.
In such a scenario, SMS commands are the only way you can still send AAT commands to your phone. If web commands don't work, you might want to use SMS commands instead in order to tell your phone "Please send data to the web site". Not only location tracking, but take a photo and backup data too!