Application interoperability is a big problem in the iOS ecosystem. This looks like a great solution - I'm surprised Apple hasn't released something like this with native OS support.
If Apple decides to levy a new API for doing interapp data sharing, doesn't this put Ink in a rough spot? Like, especially if it comes down from on-high that this is now the only way apps are allowed to talk to one another?
Second, people use products that are made by a variety of companies, not just apple. Being able to send content between apps and between devices requires an independent, multi-platform approach.
Yes, it's a concern, because Apple hasn't made your product core functionality yet, and no, it's not a big deal, because your larger product roadmap sounds like a platform for sharing content between apps on different ecosystems.
Looking for an acquisition by Dropbox?
EDIT: Don't get me wrong--sharp product, and good luck to you!
I'm quite excited to see this though - the end user experience looks great.