I've found that many iOS applications implement their own syncing that works quite well (and the OS-level syncing of contacts, calendars, and mail works well). The mere presence of an iPad means I'm more likely to use that than my iPhone, saving the battery for calls. And finally, I'm not sure a tablet with a big phone-sized wart on the back is a net space savings, considering you're usually going to want your phone closer at hand (pocket) than your tablet (bag) anyways.
"Many" !== all. It relies on developers to roll their own syncing. On the other hand this product would mean literally anything on one device is available on the other, regardless of network availability.
On top of that, it is undeniable that one of iOSs strongest points is its lack of hardware diversity. You dont expect the sync your iPad with your droid, but should you expect to sync everything between your Galaxy Tab and your Droid X? If so, whose burden would it be? Samsung? Motorola? Google? the community? Or every developer of every app? This streamlines it a bit more. And since it only relies on the location of a couple of plugs, any oew could make it hardware compatible.
It makes the tablet marginally thicker, but the lack of necessity for a second charger already makes up for that.
I do not think the design is without its flaws, however I really do think that this is moving more towards the future than a dead end.