Same as with Bitcoin itself: then you do not have to trust any third party to tell you what information has been confirmed on the blockchain.
It depends on what you mean here. Do you mean Blockstack apps? Or apps (more like protocols) developed using the "virtual chain" concept?
If the latter, then it depends on whether the protocols are designed to "talk" to each other.
If the former, then you might be misunderstanding how Blockstack works. Every Blockstack app is built to rely on the Blockstack virtual chain to link names registered on the blockchain with information in the name's zone file. These apps can "talk" to each other "trustlessly" if a) they are designed to talk to each other, and b) if they are designed to rely on the user's own trusted Blockstack node exclusively. If any third-party node is relied on, then the design ceases to be "trustless" since there is a trusted third-party involved.
Let me know if I misunderstood your question or if there's anything else I can clarify.