10 requests per second doesn't sound majorly high. That's 36,000 pages per hour which whilst big, doesn't sound too high, especially for a site as popular as SO (Alexa puts it at 137rd most popular site; granted, Alexa isn't the most accurate).
"and when Google hits thousands of pages in a few minutes, that can kick off a lot of background work, such as rebuilding related questions. Not expensive by itself, but when multiplied by a hundred at once.. can be quite painful."