McGuire's steal of third base probably wasn't just a slow guy suddenly deciding to take off because he felt like it. There was one out, at the bottom of the order -- maybe the best time to swipe third and set up many more possibilities to score than on an extra-base hit: single, sac fly, sac bunt, grounder (to at least first, second and shortstop), error, wild pitch/pass ball/balk, etc.
That was a great swimming slide by Reese to make it, too.