http://us.battle.net/d3/en/forum/topic/7923723114#16
Following was my response to the oft-asked question about Diablo III offering an offline mode (with separate offline only characters, no achievements, etc).
As much as lot of us (including myself) would like to see an offline mode (sans AH/RMAH) separate from Battle.net (great for killing time for example if you are flying, when you just want to play without having to worry about any lag, etc), it’s likely not going to happen.
Back in 2010, the plan was to have exactly this; a single player offline mode. Remnants of this exist if you look at your agent client logs where there is a boolean value for “supports_offline”. It is of course non-functional because there is no offline version of the gaming engine that it can connect to.
http://blues.incgamers.com/Posts/1/1/4/10/15061/single-player-is-online#postId_40616
But even back then, it was also clear they preferred to have the player base create their characters online (see Bashiok’s last paragraph in the above).
Then in August 2011, Bashiok explained the rationale for why they decided to completely ditch an offline mode in favor of the server/client architecture.
http://blues.incgamers.com/Posts/1/1/4/10/107630/no-internet–no-diablo-3#postId_279920
The “intense security” concerns he alluded to is of course related to the fact that the game client would have transactional capabilities (like an RMAH attached to it in some regions). Furthermore, they were able to achieve that previous desire of having the player base’ create their characters all online.