It would make sense for them to offer a dev plan with every production plan though, and let you upload your site, and serve it to a limited no of people as a testing ground. Given their 'Power Tools for Power Users' stuff on their website, they really should consider at least offering staging servers for testing.
I guess at present they're just not a good setup for developers trying to modify WP, and are instead trying to capture the market for hardened WP hosting for mostly vanilla installs.
I don't know of any large WP installs that would need the benefit of outsourced WP hosting/experts that use 'vanilla installs'. They may exist, but I can't imagine they're a large portion of the market. Perhaps my scope of experience is too narrow in this respect?