I consider it as a good thing. Those spiders index your webpages so that people searching for certain info will arrive at your website. Your website will be exposed to larger audiences. If you wish to prevent them from crawling and indexing certain files or directories for some reasons, you can use a robots.txt file http://www.robotstxt.org/wc/norobots.html and here is a validator for robots.txt to check for syntax errors http://tool.motoricerca.info/robots-checker.phtml