The only advantage of having a subdomain is if you're main domain is already powerful or you're simply looking to separate content and there is some search volume for the [subdomain].[main domain] term.
For example, if you wish to separate content by location, you could use:
brazilian.football.com, russian.football.com, etc.
or, if you have a main content rich site and you wish to separate an e-commerce store from this.
For example: ipod.com (main domain), shopping.ipod.com (subdomain = e-commerce store)
In most cases, I'd just use a subdirectory, but really depends on what you're trying to do.