Both Slack and HipChat - and probably most competitors too - have these tasty free plans that have some limited amount of searchable message storage. Fine. This gives you a free way to get started using a chat tool. Okay so far.
Now, here's the problem: At some point you and your team will get beyond the free limit and you'll ask management to pay for the app and they'll look at the pricing and not quite manage to pay up. Everything keeps working on the free level, so it all looks okay, but what is happening is actually pretty bad for your business.
You've got limited message storage. Each new line of chat means that some previous useful how-to or solution or web address has disappeared forever (unless you pay up later). So, you know that host name or phone number or set of steps is stored somewhere in the Slack, but it has gone. So you ask that question again, and get answered again (with time wasted in the process), and so it goes on.
Losing this information is costly. There's an unwritten contract that stuff in chat is still there, and we've violated that. We're keeping water in a leaky bucket and people sort of know that but forget. And having a chat system handy stops people putting stuff in searchable email or a wiki or confluence or suchlike. So, this is a double fail now.
Chat without history is expensive and time-wasting. Beware. Get a buying commitment early.