Clearing out a full transaction log is a common problem. A quick search will find you dozens of forum entries and blog posts. Because of that I’m not going to talk about the correct methods of dealing with a transaction log full error. What I want to discuss is why you shouldn’t use the following method.
And Kenneth also hits the one legitimate use: dire emergency. If this is a normal part of some process (e.g., warehouse loading), bite the bullet and either live in Simple recovery mode (understanding the risks) or get the disk space to keep it in Full mode. Switching back and forth—especially if you aren’t taking full backups immediately after switching back—is a good way to get yourself burned.