I don’t find it offensive, really. Stupid though. As someone touched on in a previous comment, you gain much more insight from actually speaking to stutterers. The maximum insight you could gain from this assignment is “oh, people get really uncomfortable and some laugh when they come across a stutterer”. And that isn’t very hard to imagine without actually pretending. Plus, you don’t feel the anxiety yourself when you’re pretending, because you know that at any point in the sentence you can just stop stuttering and finish the word. Stutterers can’t do that. That’s the whole thing: a never-ending battle. If you pretend, it’s finite.