When developing using BDUF log files are often used to determine expected input and output. The log file is a valuable tool that can be used to determine the source of bugs and problems.
However, once you step into the world of Test Driven Development you should leave the log file in the past. Instead of depending on a log file, the test suite should test for boundry cases that would appear in a log file.
BDUF is walking (or crawling), but Agile development is running. When you want to run, leave the crutch behind.