You are a few versions out of date here -- Mac screenshots now show a preview by default and clicking on the preview opens it for annotation. This is true as of 2018 (Mojave).
Ah, I have to select "preview". Thanks! That is nice to know.
Now, pardon me, as the highlight tool isn't working, everything is a rectangular selection no matter what I do.
Right click, "text and icons". Thank goodness for that[1], because apparently I have to first click what I thought was a stylized 'A' (or a wishbone), then a separate toolbar appears, that, ironically, doesn't have a way to highlight things. Go figure.
It does appear to be far more powerful than the snipping tool.
I'll still complain about the greyscale record button.
[1] With just icons I probably would have, quite literally, never clicked the markup button and just assumed the preview tool couldn't do any annotations. Holy cow that is a bad icon.
Edit: OIC, the markup button only does something when viewing PDFs, but the button pretends it does something (is clickable, changes state) when viewing image files. That is... distinctly not good UX.
