My 2p worth
As a user with no custom columns I would be grumpy about having to switch tabs. Particularly given there is no other way to add/edit comments when they are missing from metadata - it is my #1 reason for using that dialog.
I also use the ability to remove formats and (infrequently) grab a cover from a format when I am too lazy to google for one. So I use the functions others here don't, haha. I find it faster to remove formats that way than drill into the remove menu because of the E keyboard shortcut, and the fact that it only lists the formats I have of the book, not a dialog of every possible format in the library like the menu option does.
The spacing however on the new dialog visually just looks wrong to me - vertical spacing of the textboxes on the LHS in particular. I would much prefer a conventional spacing and have a blank space at the bottom LHS. Personally I would have swapped the location of the image and comments on that dialog (so image is back where it used to be) as comments need the most space but that's just me.
Incidentally do people realise you can drag/drop formats and covers without even using the edit metadata dialog? Drop the cover on top of the book details image on the rhs - I use this ALL the time to avoid opening the edit dialog. I've been tempted to add drag/drop for the comments text to the book details panel as well because my workflow is so frequently to drag/drop a cover from FantasticFiction or other site, then copy the text and paste into the comments in the metadata dialog.