Should you use Drag & Drop in Photos"

Drag & Drop is not a reliable way to transfer image files out of Photos, and it’s never really been supported. You won't find Drag & Drop in the documentation for Photos, because it's not there. 

The thing is, if you want to get a picture file from Photos, you have so many choices. First, you have to decide if you want the original file you imported in to Photos, or if you want to use the crops, edits, and comments that you’ve added. 

If you just drag the picture from Photos, which do you get? Nobody knows, and it may be different tomorrow.

Then, you need to decide things: Did you want your picture file to maintain its metadata, or did you want to keep that private? Did you want thumbnails, previews, or full size?, or something in between? What format would you like it in-- jpg, heic, tif, or what? The export dialog gives you an idea of some of choices you have to make when delivering a picture from Photos:

You can choose the size, in pixels (and thus the final resolution,) and the quality of the compression used. If you Drag a picture from Photos, which do you get? Nobody knows, and it may change tomorrow. 

For a certain picture on my Mac, the Info Window shows its size  as being 38MB and a dimension of 8245 x 4637 pixels. If I use File>Export, at full size and highest quality, I get the same dimensions but 33MB—so it’s been compressed some.  On the other hand, when I use Drag & Drop (today,) I get a size of 13MB at 8245 x 4637, and it looks a bit different--probably from extra jpg compression used in Drag & Drop. I’m not sure what I’d get tomorrow. 

Drag & Drop works with files in a Finder folder, because there’s really only one way the file can be. But a picture is not a file! In Photos, when you edit a picture, there is no file created until you export the picture. The edits, crops, and comments you add are stored in Photos’ database, and those instructions are used to create the picture you see on the screen on the fly! There is no file! That’s how Photos is a non-destructive editor— the original picture file is never altered by your edits. And using database information rather than saving wasteful extra copies saves tons of storage. So that’s why Drag & Drop exporting doesn’t make sense— the parameters of the file have not been decided yet. To speed things up, Photos may make temporary files like thumbnails and previews, so Drag & Drop may get you one of those. Nobody knows, and it may change with a new OS version. 

I do use Drag & Drop for putting pictures in email and texts, since I don't mind smaller images, for instance. But I check it! There's a possibility that D&D may produce the original rather then the edited version So I promise myself that I won't be surprised if I find that it does something differently than I expect.