@Neopopulas you can spend a mettle to take an extra action as well, and regain 1 mettle per turn up to your maximum, so when you've only 1 mettle there's really no reason not to spend it.
Though I think you can spend it on other things too like double your focus or training. Not sure if you can double your focus twice, and whether that would give you a total of 6 or 8, but that could potentially give you another 1 or two rounds?