Since I was young I have dreamed of a "boosted" snowboard, with a snowmobile-like tread that came down the middle and would accelerate me in slow areas. The weight and noise of an internal combustion engine would have been prohibitive, but clearly we're getting to the point where electric isn't so bad.
If I'm doing snowboard runs all day though, I want to show up with a handful of battery packs and run through a few in a day.
The same would be fun with Boosted. Not being able to replace it means I can't buy a handful of batteries and take them somewhere for a day of uninterrupted riding up and down my favorite hills.
Charge dead packs in the car while you ride and with a few packs you can manage a continuous day of fun. That's not possible with non-removable batteries, and customers willing to pay for all that are good customers to have and indulge.
If I have a fully charged board and 5 friends, I feel like "OMG look at how cool my $1300 board is" is going to turn into "oh it died, sorry guys, more fun tomorrow".
I think that even if people don't buy the spare battery, knowing it's an option will help them feel more comfortable with the $1300 purchase. If the current range falls just short of what you need, an inability to simply buy a second battery to keep in your pack is going to be pretty upsetting. And always being able to keep one topped off at home means that after your commute you can still go out and enjoy your board.