The most basic argument for me would be the time loss. I doubt it could be completed in less than a month or two. Why spend the time when there are soooo many good bug tracking systems available? Give me an example of a feature that you have to tweak and is not readily available.
I think a good bug tracking system has to reflect your development process. A very custom development process is inherently bad for a company/team. Most agile practices favor Scrum or these kinds of things, and most bug tracking systems are in line with such suggestions and methods. Don't get too bureaucratic about this.
