Yay, somebody else brought up the topic!
I've been wondering about D:DC's SP beta; normally I'd say giving us access one week before release would be good, but that'd be right in the middle of when I want to focus on (let's) playing Shadowrun Returns. Dilemma inbound.
And yeah, if the beta were restricted to pre-purchases (note: not pre-orders), they could gradually open up the single-player chapters, but as it stands, doing so would be borderline suicidal. Well, I guess they could give pre-purchases a separate demo key that is single-player-only in addition to the key for the semi-public multiplayer-only demo that is out now.
However, knowing myself, I usually stay far away from SP betas (and the respective KS reward tiers unless they also offer other stuff) as I replay games way more seldom than I'd like to to begin with. Giving us access to a significant but incomplete chunk of the game would be even worse, as it'd be a huge amount of spoilers with no pay-off. Therefore I'm perfectly content with eventually getting to play the first chapter in advance just to get a feel of the RP part and the characters.