Search results tend to have a low signal to noise ratio.
As a community we're effectively subsidising these services. Use of bandwidth Proxy server logs suggest that browser access to search engines is a major bandwidth consumer. No hard figures on the percentage of requests and bytes transferred from our WWW servers to robots, though monitoring software starting to become available - e.g. robocop.pl . This is not particularly effective for finding academic material on a European or global scale, because many countries only operate a two level naming system - e.g. inria.fr . These can be primed with high quality information by feeding them with URLs from hand catalogued resources, something which is being done by several of the UK's subject based information gateways.