The only foreseeable reason for having multiple domains is really for brand protection. I owned a domain that had .com .net & .org. Someone purchased .me & attempted to build off my sites success. Long story short, it lasted only a few months when members caught on that they had one of those terrible fake "facebook-esc" websites.
There wont be much wait in other URL's since the content that is "crawled" will be the main sites domain anyways,