draft: yes option for
arara is the same as calling
pdflatex -draftmode file
which has the effect of doing everything (macro expansion, command execution, writing to auxiliary files) except producing the final PDF file. This saves some time (not much, actually) because it reduces the number of I/O calls.
draft option for
\documentclass has a very different nature. For the document class itself it doesn't do much, because for the standard classes it just sets
\overfullrule to 5pt:
but it is passed to every subsequently loaded package; if a package understands it, it will take the appropriate action. For instance,
graphicx will disable graphic file inclusion; it will just look at the bounding box and will produce a rectangle with the file name inside. Similarly,
microtype will be completely disabled.
Note that the
-draftmode command line has nothing to do with the class option. So if you run
pdflatex -draft file (the command line option can be abbreviated) and the document loads
microtype, it will not be disabled.
Note also that issuing the
draft class option for better seeing the overfull boxes should be accompanied by the
final option to
microtype, lest you don't want different line breaks, which would make
draft completely ineffective for its purpose.
When to add
draft: yes? Never, but if you'd like to save a few seconds, on all calls to
pdflatex except the last one.