Just playing Devil's advocate ever so slightly (I have Erkenbrand in my all cavalry list and use him) but I would disagree that without his upgrade the all cavalry concept is useless.
I would agree that if you are building a Rohan army, that upgrading your cavalry by taking Erkenbrand makes sense at his points, just like upgrading your warriors makes sense by taking Grimbold. Pretty standard. As SuicidalMarsbars says, pointless not to take him and his upgrade.
However, I would argue the +1 fight with the cavalry upgrade isn't the linchpin on which an all cavalry force depends for Rohan.
The strength of the Riders of Rohan from my experience is maneuverability and therefore using it to choose who you fight and when. You can dictate the pace of the game. Secondly, it's the extra attack on the charge. I'd take an extra dice in the fight over a higher fight value all day long. If you can guarantee the charge and pick your targets so you have more dice in each fight than your opponent, then that is a significant advantage. Coupled with their range and bow fire, this is always possible. You can pick off enemies, force them onto you and choose to charge when you feel the odds favour you. You are in control (my experience so far!)
And most significant of all in my opinion is the knockdown upon the winning of the fight. The doubling of your 'to wound' dice makes fielding cavalry very worthwhile, if you don't try to play them like infantry or simply go all 'charge of the lightbrigade.' Your force can become very killy very quickly and you can really hurt you enemy with well calculated charges.
With the moving and shooting penalty, i think it's worth taking 1/3rd outriders. It appears to me to be very difficult to commit all your riders to a charge at the same time anyway and it is better to instead cover them with riders that can counter should your own be countered or you cannot heroic move them away.
Since much of the required tactic is movement and skirmish, outriders are for winners. Combine that with the relatively small force you have to take when going all-mounted, their special standfast rule can make them key if your force is broken and spread around the board (objectives/targets)
So whilst I agree that not taking Erkenbrand's doesn't make sense (unless all your opponent's are f5+ ...elves for example) I would actually argue that the success of the all cavalry force comes from -
1) Ability to pick your fights (maneauverability/range)
2) The extra dice on the charge
3) The knockdown upon winning the fight
That's just from my own limited experience with them. There are many things I haven't encountered yet, including coming up against other cavalry or masses of enemy bows but in the encounters that I've had, it hasn't been the +1 fight that has had the biggest impact. There is a 1/6 chance that a duel role will be drawn so one in six fights will be decided by fight value.
(I'm not saying fight value isn't important. It is and has played a large part in all of our games, allowing us to rib Hashut for his statement that it doesn't... But I wouldn't say that Erky's upgrade or a lack of it in fact prevents Riders of Rohan from being effective. Besides, my 500pt all cav rohan currently includes 16 riders of rohan. The upgrade costs the same as a mounted RRG, or just less than a rider with throwing spear...)
One for everyone to ponder!
My favourite Rohirrim hero I've played with so far is Eomer, KotP... really feels like a machine, 4 x str5 on the charge... it's a nice feeling having a powerhouse of a hero
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