The quesion is what is your company paying you to do? Is it to write software that only you will use? Obviously not. So the only way you can justify the time and expense to build a bug tracking system is if it costs less than the costs associated with using even a free bug tracking system.
There well may be cases where this makes sense. Do you need to integrate with an existing system? (Time tracking, estimation, requirements, QA, automated testing)? Do you have some unique requirements in your organization related to say SOX Compliance that requires specific data elements that would be difficult to capture?
Are you in an extremely beauracratic environment that leads to significant "down-time" between projects?
If the answer is yes to these types of questions - then by all means the "buy" vs build arguement would say build.