It doesn't usually take a too much code - usually around 20,000 lines - where I personally am not able to think my way through debugging complex failures. Tombstoning code only gets you so far (and is a useful skill). Being able to step through code and jump around symbol references is a huge help for my limited powers.

If you have fully memorized all of Linux and GLibC and all your supporting libraries then yeah, I guess you don't need powerful tools.