Agreed. Hopefully they will add this. I'm currently betting on the fact that Slack is small and it's less likely that the NSA has a direct backdoor into their systems. I would bet a lot of money that the NSA has a backdoor directly into MS Skype servers. I even hypothesized that they paid MS to purchase Skype from Ebay, because right after the sale MS changed from peer-to-peer to client-server.

Didn't the Snowden leaks say that Skype had been used to track terrorists?

The official reason is that "search would not work", but I don't know if this argument holds. Others can do search too. There is no need to search serverside when you have all messages on the client too.

By judging their client application, I doubt the serverside looks much better, so a direct backdoor might not even be needed.