Frontier Developments head honcho David Braben has been working overtime to defend the 11th-hour revelation that Elite: Dangerous will have no offline play option. A recent video interview with Braben includes his response to these issues; he states that it's not off the table, but it's incompatible with the way that the game handles storytelling. The game world is meant to change over time based on player actions, and it wasn't as functional without players being affected by each another.
Braben explains that the game's missions are generated in part by aggregates of player activity; if a majority of players choose to support a rebel faction on a given planet, for example, then that faction becomes more powerful, and the nature of missions related to those rebels changes. Keeping the game online ensures that these changes can be reflected in gameplay, making the experience more dynamic and engaging. The full interview also covers the details of beta testing and control setups, if you're not as interested in the fine details of staying online while playing in single-player mode.
Elite: Dangerous formally launches tomorrow.