I thought it up this morning and realized it made sense. It's a way to get a Calia Menethil to stay alive and make her relevant to Lordaeron, as well as seriously weaken the forsaken (which they need, badly) and have her somewhat allied to the Alliance. It also gives some closure on the Lordaeron story. The Argent Crusade just sitting there isn't impressing anyone. But Calia marrying Tirion would change things up a bit. I could go on about it, but this posting format isn't suitable (word limit).