The price point for the beach wheelchair you linked is $2,440.00. That's about half the price of this rig, and it does not get you to the beach. You need to have (and park/stash?) another wheelchair for that. Oh and you have to have someone push you.

By the way, I read @codekansas' comment [1] not as a warning about feature creep, but as a comment on the inertia in the medical industry (emphasis on "industry") versus just a few enthusiasts who don't feel bound by patents, conventions and markets, and simply want to build something to improve their lives.

[1]: https://news.ycombinator.com/item?id=23703179