Kiwi, I know you're not a huge GR user, and you haven't gotten too many responses about how people use GR, so just to give you more idea of my workflow with GR and Calibre (keeping in mind that although I am a heavy user of GR, I am of course just one user)...
When I get a new book, I add it to both. I had been in the habit of adding the book separately to each, as part of my workflow pattern for a new book - I kind of do it all at once; dl the book, load it into Calibre, open the GR page for the book, edit metadata (in Calibre) for the book to make sure all the fields are filled in the way I like them to be and conform to my practices, in GR click "add to my books" to add the appropriate version of the book, then immediately click "Edit my review" to add a few shelves (new books go onto the "to-read" shelf out of the three "mandatory" shelves (to-read, read, currently-reading), but I also put them into a few other of my own shelves depending on the book - e.g., fantasy. I don't have hundreds of shelves as many GR users do - I don't use shelves the way I do Calibre tags; I use Calibre for that! - but I do have 5-10 broad categories).
All this is a bit of a pain. With the plug-in, as long as I have the correct ISBN, I can streamline - just add the book to Calibre, make sure the metadata is correct, then link the book to GR and add it to the correct shelves. Though it would be very helpful to be able to add to more than one shelf at a time - it is a bit laborious to have to do them one by one (I always have at least two - to-read and at least one other).
The next time I use GR is when I finish a book, and I described my workflow in an earlier comment. That's when I normally would, in both Calibre and GR, change from "to-read" to "read," fill in the "date read" field, and occasionally add a rating (unlike some, I do this only once in a while). I might add or change a shelf (after reading I might decide different tags apply, or I might add a book to the "favorites" shelf or the "DNF" shelf). Ideally, just as when adding a book, I'd be able to do this from Calibre and then sync the info for that individual book "upwards" into GR.
I do occasionally write reviews in GR, but since I consider those for "sharing" purposes I don't save them in Calibre; I do them through the GR interface. I suppose others might like to duplicate reviews in both places, though.
I'd be interested to know whether others use GR and Calibre in a similar fashion as me and so would like the plug-in to serve similar purposes. The plug-in is certainly useful as a tool to for an initial sync of GR and Calibre, to get entire shelves full of books from GR into Calibre - but once that's done (or for people like me who already have their libraries in both places), what I'd find more useful (would in fact be abjectly grateful for!
is a tool to help make it easier to keep the two synchronized on an ongoing basis, with less duplication of effort.