Debuggers were introduced in my second year Intro to C course, if I recall correctly. Of course the problem most students were struggling with at that point was getting their work to compile, which a debugger will not help with. And once their ten line command line program compiles and then crashes, well, they already have some printfs right there. Fighting to master GDB is overkill.
In my experience, it's fairly rare to actually deal with a code base large enough to make more than a cursory familiarization with a debugger worth the time investment in most Comp. Sci curriculums. The programs are small and the problems you face are more along the lines of figuring out the time-space complexity of your algorithm.
Debuggers become much more valuable on real world projects, where you have a lot of code written by different people at different times to trace through to figure out what keeps frotzing foo before the call to bar().
