The battlefields in BD were entirely optional; I only visited for the merchants.
Lots of people wanted respawning opponents after Divine Divinity, optional areas and the ability to keep playing after the end of the main plot. It wasn't a majority, but apparently enough to justify an experiment... or it was a requirement from the publisher.
I'm typically one of those people, so I guess I can't complain too much about their existence, then! And at least one set of battlegrounds provided one of the rare examples of pleasant greenness in the game. Based on what you've said below I guess I'm surprised they're there at all, let alone the size they are.
The DD wastelands were cut back quite a bit from the original plan, due to the publisher setting the release date, leaving it almost entirely hack and slash.
With BD, the final act was done first, so this would not happen again, but unfortunately that put limits on the design of the rest of the game, since changing too much or introducing new mechanics could invalidate the work already completed.
That's a bit sad. I wonder how many games have been less than they could've been because the publisher wanted them out earning money before they were ready...
