Now that they have a web client they should consider making an official Tor service like Facebook and Twitter have. It has been used for activism in the past.
I don't know if that's irony, but do you really think that putting a service which cannot be trusted on Tor makes it trustworthy?
No, but there is a use case for people whose threat model allows for identifying themselves to Snapchat but not for being identified by their ISP or government. Access has also been blocked in various countries over the years and a Tor service restores access.
When you use a Tor hidden service you don't use an exit node, your connection ends within the Tor network. As for the internal communication attack there is no reason at the time of writing to believe one has occurred, and it's unclear if their statistical attack on Tor's crypto would work or if it is possible: https://blog.torproject.org/rumors-tors-compromise-are-great...