i think the #1 value add from slack/hipchat is that it's hosted for you. we spend thousands of man-hours each month and year keeping our servers up. uptime is hard. chat is not hard. historical chat is not hard. when chat is down, your business is at risk. there's other services at my company we pay for that aren't very complicated - we just don't want to host it. paying $5/user/mo for chat that is always up is easier than keeping it up yourself.

your use case is very different, and i wouldn't expect any pay-for-chat service to make any sense for it