For one thing, there are no handholds on the bottom of the Shuttle. Lack of a handhold means that there's no way even to put some sort of repair-goo onto the damaged spot. The slightest pressure will send the astronaut tumbling away. (To see how litte forces make for big tumbles, go find the video from the first two INTELSAT capture attempts during STS-49.)
Second, the hard toe of an EVA boot could kick a good-sized hole into the tiles -- making things worse instead of better.
Third, the patching compound itself could well make the surrounding tiles more likely to detach, due to ablation or by preventing them from flexing as they're designed to do.
I always lecture people here at my airport that pre-flighting and knowing your aircraft is essential to your survival. I've seen a mouse nest collapse the nose gear on a King-Air, a craze line in a P-Baron side window cause it to blow out during flight, an overlooked altimeter setting cause a Sundowner to clip the top of a dumptruck traveling at 60 mph at a right angle to final, left on pitot tube covers, tie-downs still attatched to tails, departing with a concrete filled tire hanging from the plane, cylinder separation due to birds nesting in cowlings, an countless more......I guess just because it looks like a plane, I expect it to be used like one and with similar management methods. Assumptions obviously on my part.