Would an Aikido practitioner learn to use various weapons simply for the sake of defending him/herself from those who would use them offensively, or do Aikido practitioners occasionally use weapons for offensive purposes (i.e. knocking someone unconscious, assistance in a joint lock, etc.)?
Answer to question one - yes!
Answer to question two - Not sure what you mean by offensive, but weapons training improves timing, distance, extension etc... It's also a good way to do moving meditation.