>You have to manage an inventory of expensive $20k+ parts that could be stolen,
All inaccessible and underground. They're also fairly useless to thieves; who would they sell a stolen Tesla Model S battery pack to?
The battery packs are heavy, unwieldy, and can't be resold to anyone. If you're a thief, there are much better targets.
>you have multiple sizes and model of battery,
All the loaner packs can be the same size and model.
> and different wear states. The batteries lose power constantly.
Since they're at the charging station, they can keep the batteries topped off. As they wear out, they'll be replaced. Tesla owns the loaner packs. The battery swap is actually a loan, not a true swap like propane. You have to go back to that station and get your original pack back.
>You have to manage liability, if you install a defective battery and it catches fire who pays.
Tesla, since they're both the manufacturer and the battery swapper.
>You have complicated machinery that you need to have many of to handle rushes that go unused at other times
It takes 93 seconds to swap batteries. http://www.teslamotors.com/bat...
They really only need one swapping machine on site for the foreseeable future, and if they get to the point where they need more swapping machines, then they're doing very very well.
Especially since swapping isn't going to be used day-to-day; you'll charge at home or work. Swapping is really only for long-distance trips.
>And you still need to have the same order of magnitude of power available to charge up the swapped out batteries as you would to just charge them in the car.
Of course. The advantage of battery swap is that you can run out your current battery, swap at the station, drive wherever you're going, come back, swap back for your now-recharged pack, and go home. 186 seconds during the trip, versus having to stop and charge for a few hours.