Personally, I don't feel he's that cost effective, since I never feel the need to cast the same power more than once. I find taking Special Characters too limiting, as they take away vital points that I need to spend on the rest of my army. For fifth edition, I think spending points on squads is much more important. My regular Farseer with Doom, Fortune and Spirit Stones is much cheaper than Eldrad, has a much more interesting story to her life (which you can all read, should you wish to, when I post my Craftworld background), and does everything that I need her to do on the field of battle. I appreciate that a lot of this maybe irrelevant for tournaments, so I'm only talking about regular games here.
As for the background, I don't quite know how Yriel got into this discussion, but I never argued that Special Character should never be fielded, or should never appear together. It's highly likely that they could in Apocalypse games or in narrative campaigns, but would they appear all the time, in a regular Eldar army, in a standard game of 2000 points or fewer? My answer is no, not unless it's part of narrative campaign or a specialist scenario. As I say though, for those who are not interested in background these sorts of issues don't matter.
As for your last comment, I assure you I've won games with my Eldar, and I haven't needed Special Characters to help me out. It is more challenging to win with the Eldar in fifth edition though, but as I said in an earlier post, I believe that this has more to do with the missions than it does the Eldar Codex and its HQ options.