Students need to find relevant information, not just anything. An assignment has to be doable. Not all topics will have indexes to articles in all formats. Don't set them up for failure or have them doing pseudo research. There are fewer and fewer CD-ROM indexes, and some indexes are only available in print or electronic format, depending on the year.
Competitive students have an incentive to hide or destroy library materials.
Very little, if anything, is learned from this kind of experience and it unnecessarily harms the library materials. Show your assignment to a librarian before making copies and giving it to your students. Emphasize to students that they must not move materials away from their locations or other students won't be able to find them. Explain that everyone needs to have access to materials. Don't grade the assignment on a curve, but on straight percentages.