I personally really like not being forced to pay extra for a big battery when I buy a new device. I already have enough batteries I carry around.

On the other hand, I think it could be a nice touch to give the customer an option to buy a case/battery combo, designed by the same manufacturer.

Yup, agreed. I have a big powerbank i use for everything at home and i see no reason to increase the bulk of the Switch itself to include a bigger battery.

Likewise if it wasn't built in but came with the product, i'd be wanting a lower priced product not including the external battery.