It's certainly something that you won't find lying around by the side of the road.
I've never had a chain break in transit, but my topeak multi-tool has a chain tool on it... I've definitely used it at home to crack a chain to clean it. Ditto on the masterlink, if you can use it, it's a good call to have one, and carry a spare, but you can be in a spot where the link might need to be forced open anyhow even if you have a masterlink.
The weight of a multi-tool, versus the weight of a multi-tool with a chain breaker is a minimal difference. Better to have it and not need it and all that.