When can spackle be used instead of mud?
You can use either spackle or joint compound. I personally detest the modern "light weight" versions of spackle and will not use them. I use joint compound for most repair applications around drywall. The dry time has never been a big problem for me as most projects are big enough that there are plenty of other tasks to attend to whilst the joint compound dries.   

Note: Most of the preferred "old style" heavy spackle is also a pain to use in comparison to joint compound because it drys really hard and is a total pain to sand down. Joint compound is really easy to sand thus giving it one more feather in its cap.