Tolkien seems to have had a hard time actually finishing things. He would go back to tinker or re-write rather than producing a single final draft. (That's intended only as an observation, not a criticism. I sympathize with the tendency.) So for the stories he really liked (of which Beren and Luthien is certainly one) there's a lot of material.

I'd hope for a completed poetic version, but as a life-long Tolkien fan, I'll be buying it regardless.

Also, with The Silmarillion, they tried to add as little content as they could and just fill in gaps between his existing writings. That's why it has such a cut-and-paste feel to it, and I expect this book to be similar. While I think it's good to get as much of the original writing as possible, it would also be interesting to see them pass the stories off to another fantasy writer.