I am going to assume it is rude, simply because I've never had anyone finish my sentence for me. Perhaps I'm just terrible at getting the jist across, perhaps others fear they'll get my words wrong and offend me. Alternatively, perhaps I've been able to improving my stuttering skills (or lack thereof) over the years to the point that it's much less noticeable now and people don't have the chance to interrupt.

Having said that, I don't think I'd mind if someone completed my sentence for me. Sometimes I forget something or it sits on the tip of my tongue and it's helpful when someone answers it for me. "What was the name of my 8th grade English teacher again? Mrs... Mrs Boooo...uuu...bro....brun....brin....?" "BUCK!", someone might reply. I don't think anyone has ever been annoyed in this scenario, when your memory stutters, so it seems the same rules should apply to when someone actually stutters.