Exactly what BC said. I don't know of a MMO game will let you log in if you're not running the exact patch the server is running.
The reason for this is mostly critical changes and balance issues. For example, think of a character having a far overpowered ability in say patch 1.10. The MMO company thinks "oops, we overdid that, let's tone it down a bit", so they lower the effectiveness of that ability in their next patch, 1.11. Every player must now upgrade to 1.11 in order to continue playing, because now the server knows that ability as being less effective than it was before, and for one person to stay on 1.10 with that overpowered ability makes it unfair for anyone who upgrades.
When playing an offline game, this doesn't matter.