Bug tracker contains a lot of code to handle categories in a hierarchical manner, if we'd use category system, we could just reuse the functions written therein. Furthermore, it would be a good reason to integrate categories with listbuilder (unless it is already done), meaning, you can map multiple category trees to a package and the listbuilder would display them automatically like the bug-tracker currently does manually.
Last but not least, it would allow us to use multiple categories within one tree, something that would break the current bug-tracker. Useful if a bug e.g. belongs to more than one package.