There's many risks involved with using a cracked theme. But another element not mentioned...the designer of that theme is losing money from having their work being downloaded for no cost.

1.Basically, piracy is bad and dangerous. Theme developers can contact your web host provider and get your account suspended, if they find that out.
2.There can be hidden and encrypted code in your theme files which can make your website remotely accessible by a hacker.
3.You will not get support and updates for the theme.
4.There may be hidden links to spam sites, which ultimately damage your search engine rankings.