In the current web, sites like Amazon are so large that you'll need many crawlers. On the plus side, it appears that almost all large sites don't have rate limits.
http://www.bing.com/search?q=%22We+have+been+receiving+a+lar...
I agree that crawl-delay is rare, and often it's set too long so that it's impossible to fully crawl a site -- as if the webmaster set it up 10 years ago and never updated it as their site got faster and bigger.