Join Date: Jan 2009
Location: Worcestershire, UK
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.