My recommemdation is never EVER use the color labeled Registraion in swatches. Only use the color labeled Black for black text. (100% Black) I know this what design classes teach you to do, but for offset printing, this is a nightmare for any four color pressman. Getting the 4 color images into proper registraion (Cyan, Magenta, Yellow, Black) is hard enough, without worrying about trying to keep a 4 color black build in the text or any small keylines in registration too. Especially if they are 100% of CMYK.
Off the top of my head, the only use for the registration swatch is to color registration marks on the off chance someone wants to add their own. I can’t image any competent design instructor telling their students to use it for text — then again, there are some design instructors who only teach design but know next to nothing about print production requirements.
This is correct.
The registration swatch is only used for - you guessed it - registration marks.
You might want to add your own registration marks to a project for whatever reason you have. And the registration mark will output on all plates or for an InRIP separation somewhere.
You don’t have to use black. It could be Pantone Colours only.
You add the registration marks.
And when the colours are separated they appear on all separated plates/film/paper or whatever you need it for.
100% agree, obviously.
I have seen designers just grab any old swatch and color artwork with it though. Most commonly, swatches designated for die lines - like a fold indicator. They would be given old production files as a starting point. I have seen the registration swatch used too. 
It should really be banished for Text - but that is not really feasible there are legitimate reasons to use it on text as you might want the text in the Slug Area to be in Registration colour.
Digital print doesn’t have the same issues with Registration black in text as plate presses.
Still…ink limits.
How’d that job go with 19 spot colours and black text in registration colour!???