The reactor in the middle of the chestplate was actually not a reactor at all in the comics (at least at first), it (and the entire chestplate) was a powerful electro-magnet, then it became another repulsor like in his gauntlets. It is left exposed (not behind a window) so he can use it as a weapon against his enemies. He called it the Unibeam. But even that is not actually his arc reactor showing through the chestplate, since the reactor in his chest is almost flush with his skin, what you see in the chest plate is a repulsor and is hard to destroy. It took super soldiers armed with vibranium or Vision's beam cannon from his head gem to disable it.
However as a reference to the US War Machine comic, Rhodes lead a team in knockoff War Machine armor when he realizes that the knockoff armor's designer had placed the unshielded power cell in the middle of the chestplate, because it 'looked cool'.
There is also the theory that Tony's arc reactor is actually based off the Tesseract, and possibly built with some of the Tesseract's elements. I imagine if that was true then it would make it quite durable as well. Possible proof for that is in a comparison of photos. The first photo being from Captain America: The First Avenger when Howard Stark is examining an element found in a Hydra base, and the other photo being from Iron Man 2 after Tony recreated his fathers 'lost element'.