My team built a couple bots that help us keep track of issues we are working on, schedule releases, etc. We also do all of our meetings on Slack; when you're on a call it automatically updates your Slack status to reflect so, that way your teammates know you are busy.

Whether people use all of Slack's potential or not it's a different discussion, but I'd rather have the ability to do more if I wanted to than having to look for yet another tool to solve a small problem.

Don't get me wrong - I'm 100% in favor of extensibility through plugins and bots.

I don't think that means that the default client can't be lean and efficient.