For undead and the like not at all, but a lot of people I know gladly pay 25 points to have a few extra skellies around.
For warriors, my current main army, they are necessary. The so-so ld across the board necessitates a BSB.
You don't always need them, but it never hurts to have one. IF you would take a fighty hero anyway then pay the little extra to make him a bsb, and if not consider it. IF you want a magic banner you have to look at it a little more carefully I'd say. Only for specific builds, otherwise it is a waste.
Basically its either an auto-choice, or a non-choice. There are very few armies where having a BSB is really a middle ground...
VC, TK and daemons basically don't care about BSBs. If they've taken one it'll likely purely to carry a magic banner... they certainly won't include a BSB just as a BSB.
I think Skaven and Lizards are about the only armies where a BSB is kinda useful but not really something that should be fitted into every list: skaven tend to get Ld10 on almost all their units while cold blooded means that even at Ld8 lizards are pretty hard to shift (play a Ld9-10 slaan and you really don't need to worry).
The 'high' ld armies such as all the elves and empire (with Ld9 lords) don't absolutely need a BSB... but Ld9 is still a 1in6 chance of failing, which is pretty terrible for the more elite armies.
The low-mid ld armies almost require a BSB to be effective. There armies include ogres, OnG and beastmen but also armies you wouldn't normally think of such as WoC (although less then the others). These are the armies where players cringe everytime they have to make a panic check or are horrified at the thought of making a steadfast check. Even the strongest Ld7-8 units are going to hate the thought of taking a chariot/monster to the flank: they will likely lose, could well break, have very little chance of reforming and generally won't do well.
For pretty much all of these the reroll from BSBs is well worth the 25pt cost, the only question is can you fit in the hero into your army in the first place to carry the BSB. For my ogres that's a real problem as characters are all very expensive... but for other armies where you can pick up a decent combat hero and add the BSB in for ~150pts its well worth taking.
Tim/Steve - Disagree on the Daemons bit, I think they are a must as we dont crumble like Undead, we lose an amount of models = to how much you failed the Ld test by. Although, my daemons do have a habit of spooning in combat and end up losing by a lot
Back on topic, i find that all armies could do with a BSB except those who suffer the undead rule.
Picture this - your death star unit, lots of points sunk in to it, you have all these amazing buff and you have the stubborn rule - you fail your leadership by 1. those 25 points just saved you a thousand points, or saved your right\left flank. My books - BSB is a must.
There's no way in hell I'd play a Skaven or Daemon army without a banner. A 25 point upgrade that can save entire flanks, as someone pointed out earlier? Why would you NOT want this? A Nurgle herald with a magic banner and a magic level might be a bit points-heavy but he can lay down the smack and keep his army in the fight.
Skaven may get high Ld from big blocks of troops but if you start failing break or panic tests, you might as well sweep your army into the carry case and go to the pub. As for Daemons, the effect might not be so dramatic but it could mean the difference between losing half a dozen models or even a Greater Daemon.
With the new rules daemons pretty much must take a BSB. Losing a combat by 1 and then rolling 6,6 would mean you're whole unit disappears... while with a BSB you have almost no chance of that happening (1in1296) while having a much better chance that you unit recovers all lost wounds.
Since heralds are going to be in every daemon list anyway its not a huge cost to just add the BSB. Sure you can't just throw on magic banners any more without changing your build (and lets face it, daemonic gifts is why you take the herald) but just adding a plain banner is an almost laughably easy decision.
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