Someone else's perl is a lot easier to parse than someone else's compiled C/C++ code. In addition to all the reasons already mentioned, I like to use perl because of it's inherent interpretedness (did I just make a new word?). I've inherited a lot of code from a lot of people, and it makes my life so much easier to be able to point to a script and say "HERE is the source code" than to point with one hand to an executable and the other hand to some C source and say "I *think* this is what's running..."
Heaven forbid you inherit the executable but not the source. *shudder*
Heaven forbid you inherit the executable but not the source. *shudder*