> Why would anyone use Polonium 210 as a poisoning agent?

It's a very rare agent that only very few assassins have access to, so most routine poison searches would not pick it up. Especially since the symptoms are basically nondescript. In addition to that polonium was pretty much unknown as a poison when Arafat was killed - the high profile Litvinenko killing was after Arafats death. The lethal dose is extremely low and even if it is detected, there's no antidote.

That's a pretty good stealth poison in my book.