Sometimes you just have enough runs. Going out to bat facing 500-odd means the WI bats feel the pressure of some chance of getting back into the game which creates more chances for the bowlers and gets them back in the sheds sooner if they take advantage, keeps their mind on taking wickets. Facing 7002452340+, the game is dead, the bats are just going to bat time because they're out of it and bowlers have to work harder for wickets ("God, why doesn't he just get out already?!" The game's done."). Batting less here makes it more likely Voges won't be shot in a game down the track too.
So yeah, I back Smith's declaration, I'm guessing it's just him maintaining the style in which he wants the team to play because letting things get too flabby mentally is dangerous, indulging for selfish reasons doesn't promote good habits. I mean, if you're maintaining a good fitness level and you're confronted with a buffet of all your favourite foods, do you gorge? No, you show some discipline because 'just this once' can go down a bad road. It's not an #intent or aggression thing, an early declaration keeps things moving in a team, fitness and positive psychology sorta sense. Has nice flow-ons when you play against someone good.