Disadvantages
Despite the use of the terms "allow" and "disallow", the protocol is purely advisory. It relies on the cooperation of the web robot, so that marking an area of a site out of bounds with robots.txt does not guarantee exclusion of all web robots. In particular, malicious web robots are unlikely to honor robots.txt.
While it is possible to prevent directory searches by anybody including web robots by setting up the security of the server properly, when the disallow directives are provided in the robots.txt file, the existence of these directories are disclosed to everyone.
There is no official standards body or RFC for the robots.txt protocol. It was created by consensus in June 1994 by members of the robots mailing list (robots-request@nexor.co.uk). The information specifying the parts that should not be accessed is specified in a file called robots.txt in the top-level directory of the website. The robots.txt patterns are matched by simple substring comparisons, so care should be taken to make sure that patterns matching directories have the final '/' character appended, otherwise all files with names starting with that substring will match, rather than just those in the directory intended.
Read more about this topic: Robots Exclusion Standard