Most of what has kept me interested in RP has been because of this in the last few years. I could trawl the old forum archives for more, but without this, and the precursor form of CrossRP, I know many people wouldn’t be playing.
Illidari guilds can exist cross-faction, Argent guilds exist, Dalaran has a thriving, regular RP community, Booty Bay can also have the sensible cross-faction pirate theme. There’s really nothing negative this gave in my opinion.
Some ongoing/upcoming examples of what couldn’t exist in the form they do (or at all) without them as well.