In the Litvinenko case it was probably used as a signature, since there are very few entities on this planet that can actually obtain a sufficient amount of the isotope. The assumption is that the assassin wanted everybody to know who gave the order.
That same reason also makes me somewhat sceptic in this case, but who knows. Hopefully, time will tell.
With Arafat, the assassin almost got away with it unnoticed. There could be other cases of Polonium use as well which were never reported, it's remarkably tough poison to detect.
That same reason also makes me somewhat sceptic in this case, but who knows. Hopefully, time will tell.