EDIT: Soul Reaver, a Rogue Trader adventure supplement, is rumored to have rules allowing players to create and advance Dark Eldar Kabalite Warrior Characters, and these could very well be used as the basis of home-brewed Eldar Character creation rules.
Did we really need a fifth "stand-alone" game, or was it just a case of "we'll get the core rules banged out in a concise and comprehensive rendition sooner or later"? Going into the fifth redesign of the rules do we really need to be asking them for a sixth? I'd rather have supplements than rules. I already have enough rules to choke a hippo. Actually, I have enough supplements to choke five hippos, but that's not the point. Sure, people will buy an Eldar-based game. People would also buy a Tau-based game, an Ork-based game, and/or a Squat-based game. A supplement the size and price point of the Inquisitors Handbook could easily cover Tau and Eldar, and additional supplements could be released focusing on those PC races…but the question remains- does FFG produce yet another $60 core rules set, or get the rules situation ironed out with Only War and put the following on the back of each Tau/Eldar/Ork/Squat release: "A copy of the Only War Core Rules is needed to use this supplement."
As an aside; if FFG pulls another copy/paste stunt like they did with Hostile Acquisitions they're gonna find me standing on the front door of their offices, waiting to personally ask to have my money back. That was a lame release. Lame.