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The electric commuter/urban runabout has existed in European and Asian markets for years (see also: G-Wiz). Tesla is attempting to compete with the sedan and sports car market, which means that they need to be able to match the versatility and convenience of gasoline powered cars.
I think you're underestimating how much the battery packs in these cars weigh. Castors or no, the system is going to have to be automated, which adds considerably to the cost/complexity involved. Making the battery easy to remove and replace is also likely to require a form factor that will either substantially increase the size of the car or result in a trade-off in cabin space.
I'm not saying these are insurmountable issues, but this is the major hurdle with a system like the one you're proposing. It would require a lot of people to commit to developing and instituting it in order to be successful. I don't think it's likely to get that kind of support, and I see hydrogen as a better answer because it's more of a drop-in solution. It also has it's problems (mainly storage/transport, but also on the production side), but they seem more easily dealt with.
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I wake up in the morning more tired than before I slept
I get through cryin' and I'm sadder than before I wept
I get through thinkin' now, and the thoughts have left my head
I get through speakin' and I can't remember, not a word that I said
- Ben Harper, Show Me A Little Shame
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