Just because we haven't seen or heard any issues with the factory stock impellers/water pumps, doesn't mean they can't become an issue or either nobody has reported any on the interwebz. If I was already replacing the thermostat and temp sensor, IMO it's smart to change the water pump as well since you're already working in that area. Doesn't cost much and not too much added work involved...better safe than sorry.
After taking out my stock water pump, I placed it on my garage floor vertically and it lightly fell on it's side, the plastic impeller broke easily. It was pretty damn brittle after 73k miles if you ask me. Glad I replaced that part.
