In the NYTimes interview with Butterfield, he was clear that the money is because it's so easy to raise money right now.

Having a huge battlechest is useful for being able to try risky new verticals without risking future growth too much. Like how they purchased that video chat startup and integrate that with Slack.

He also talked about having cash to be able to give more to employees in the various situations, since stock options are useless in the case where Slack won't be public for 20 years.

To be a Ponzi scheme (which I assume is what you mean), a key factor is massaging new investments to look like revenue, and using it to pay earlier investors. I see no reason to think that Slack is doing anything like that.

"Venture Capital"?

(I know that's not the P word you're going for, but alas...)

I always thought it was close enough in many times in practice. ;)