I disagree about the MS part. I think Slack would probably do very well there. The enterprise market is where Slack is going, and MS is fully entrenched there. They also have a very developer-centric culture that some people enjoy.
I don't necessarily see why MS would purchase Slack instead of delivering their own solution. Unlike Skype it's not really key to have a large-user base on the service, all that matters is that your entire business is running the same solution - and Microsoft already makes and sells other related enterprise tools which gives them an advantage both in terms of sales and integration. Perhaps it's even more surprising that they haven't already delivered a competing solution.
Yammer is a team communication solution Microsoft already bought, but is way worse at synchronous communication (chat) than Slack, but also better at at asynchronous communication (threaded conversation, like on Facebook).
Microsoft would want to buy Slack for the same reason they spent $$$ on best-in-class products like Yammer, Sunrise, rebranding Acompli as Outlook. Slack's revenue is incredible for a startup, but a drop in the bucket for MSFT (for now). More immediately, Slack represents an entry point for Microsoft to upsell Slack's customers to other Office365 services and Azure services.
