Actually, in the TK faq, they did clarify the Casket (and for that matter the Banshee as well). Anytime the general is within 12", all models are considered to have his LD, for any purpose. As for the Blade, it says "unmodified LD", and this clearly means the model's own LD, not the mount's rider's, not his general's, etc.
I read this to mean it could not benefit from the general, or even a rider if it were a ridden monster. IMO this is now the best weapon in the game, even though it's priced just high enough that you can't get any nonstandard protective items to go with it... The potential here is devastating if you can keep your guy alive. People have pointed out that he will lose combat against a large, properly equipped block of troops... of course he would, alone. I'm probably going to put this guy on a carnosaur and run him at enemy characters riding on big crazy monsters. His blade will drop the monster outright most of the time, and the carnosaur should have little trouble with most of the leftover riders.
I am totally in agreement with Daeghrefn here... this weapon is able to do things that will force the enemy to either severely alter his battle plan, or else lose some of his big monsters outright. It's killing blow without all those pesky limitations that keep it balanced. Against leadership 9, you will kill anything outright 1/6 hits. No size limit, no wound limit, no pesky ward saves or wound rolls. Even against the enemy general, who will typically be Ld9, it's a 1 in 6 shot at (better than) killing blow per hit. Some quick Ld 9 math... 1 hit 16.6% chance 2 hits 30.6% chance 3 hits 42.1% chance 4 hits 51.8% chance 5 hits 59.9% chance So even against high Ld stuff this is something that will strike fear into the enemy. Assuming you are hitting on 4's and pulling 2.5 hits out of it, you're looking at insta-killing better than 1/3 of the time. Not too shabby if you're talking about a high value target. Hit him on 3+ and it gets closer to 45% chance on average. In the case of Ld 10, I would say you can't rely on it too much, though we've all seen even Ld 10 fluff a roll or two. For Ld 10 make sure you can handle your target without ripping its soul out, or use something else against it. Don't think of this weapon as a rank-and-file killer, it's closer to artillery. It should punish those models with the highest point cost first, making a mockery of stacked defenses. Sure, giants have Ld 10, but they'll get ripped up by the Carno and its Str7 D3 wounds along with the 5 str 5 from the Oldblood.
you can't always rely on mathhammer, but it is a risk thats worth it's crap. it feels sweet to get rid of an unclean one on the charge.
An old-blood on a carnasaur with the blade af realities is truely devestating. It has started to persuade me to have a saurus oldblood instead if a slann mage priest, or maybe i could have both of them!