Checking an ID is a way to double check the credit report and references. A landlord needs to confirm that his applicant is who he claims he is. The credit report that a potential roommate provides you must be his. Do not accept a copy. A copy is easier to tamper with.
Nothing is foolproof but seeing the original helps.
Also ask to see a social security card. Compare this with the name and social security number on the application and credit report. Everything should match. Roommate Locator Inc.