You should use point to point message when you want an reply (acknowledgement ) for a message from the Client. In case of pub sub the acknowledgement is sent by the InstaMsg server to the Publisher,on receiving the message, and not by the Client (Subscriber).
Use point to point message when you require message acknowledgement between publishers and subscribers. For example:
Device Signaling
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Signaling Uploading and downloading files on devices.
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Changing configuration on clients or devices
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Upgrading firmwares etc.
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Activating digital outputs etc.
You can find more details here http://www.instamsg.io/docs/messaging#instamsg-docs-developer-guide-comparison-messaging-patterns.