We all love the idea of lighter, but for like a 4% weight saving for 40% (based on 840 would give you 40% more than 600) less range/altitude/Turbo nutter power....Yes, when buying it would be nice to choose what you prefer, 600Wh or 840Wh..
How many people in the real world will actually go that way ?? Add in that the lower your standard bike spec, the less % weight saving on your Levo SUV (due to the bike being heavier to start with).
Even if you work out that you personally will probably never use more than 600, it's always nicer to have more if you do decide to do something different and when/if you come to sell the bike, people are more likely to buy the one with an 840wh than a 600wh on the second hand market.
OK, there could be exceptions, like someone who'd like two 600's and swaps out back at the car at snack time rather than the 840 and the range extender so they can still have a water bottle. (Almost looks like there could have been space for extra bottle bosses higher up so you could go super lard with 840, extender and a mini fidlock)