Although the Far Realms are very Lovecraftian, you don't necessarily need to bring his Dreamlands along for the ride. In fact, I probably wouldn't in a Far Realms themed campaign.
That said, I think the real benefit of using the Dreamlands is getting to use Leng.
The Plateau of Leng is a plateau and impossibly high array of mountains that exists primarily in the dreaming world, but also intersects with many real world locations (generally in high mountains, like Tibet or the Storval Plateau in Golarion). The original inhabitants were a race of spider-like beings who were overthrown by a race of twisted men.
The men (called Denizens of Leng in Pathfinder) make for pretty cool villains. They're mostly traders outside their native environment, so some wouldn't find them objectionable. They're great to have be behind the scenes of various organizations, funneling them money (in the form of large, blood red rubies) or magic. They expect slaves and favors for this bounty.
It's never explicitly said, to my knowledge, what they use the slaves for, but I assume they take them back to Leng and slowly twist them into more denizens. On a similar note, I like the idea that they can capture people's dreamselves and use that connection to slowly, horrifically warp their mind and body.
Anyway, if I were to use the Dreamlands in your game, I wouldn't use them as part of the Far Realms. In Lovecraft's works they're the home to the gods that are most like humans. You could have the Dreamlands serve that purpose there, instead of the normal set of planes, and have Leng be the Far Realms' beachhead in the gods' domain.
Cheers!
Kinak