Hi Franklin,
It depends on what attribute you're trying to set. Using wf:set on a form element will change its value. Using it on an image will change its src attribute.
You'll need to update the value with jquery, which can be simplified a bit with the objs(ID) javascript function:
wf:wire("objs('elementid').prop('value', '15)")
That said, modifying the #set{} action to support an attribute attribute (heh) might be a good idea:
wf:wire(Id, #set{attribute="some value"}).
And making a separate wf shortcut call for it like
wf:attribute(Id, "Some value")
Might not be a bad idea.