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You probably won't get very many banner triggers, but you get a different benefit: your opponent wastes attacks.
As far as I know, your opponent has to declare where all of a unit's attacks are targeted before rolling them. He or she can easily figure out how many attacks on average it will take to kill your unit, but that's just an average result -- half the time it will take more attacks to get the job done. So your opponent then has to decide how sure he or she wants to be to get the job done. The more certainty, the more attacks need to be allocated.
For example, let's consider crap quality attacks (4+/4+/rend 0). Using just the basic 5+ save for the moment, each of these attacks has a .139 chance of causing a wound. On average, it will take 35.97 such attacks to deal 5 unsaved wounds. However, fully half the time that many attacks will actually fail to destroy the unit. If my standard deviation calculations are correct, it will take 54.315 attacks to have a 66% chance to cause 5 unsaved wounds and fully 80.6 attacks to have a 95% chance to cause 5 unsaved wounds.
For another example, consider more elite attacks (3+/3+/rend 1). These attacks have a .3086 chance of an unsaved wound. Here we see:
50% chance: 16.2 attacks
66% chance: 23.45 attacks
95% chance: 33.54 attacks
If you want to shorthand it, the pattern seems to be that it takes approximately 50% more than the average number of attacks to hit that first standard deviation and 100% more to hit two standard deviations.
When you use multiple small units and your opponent is highly incentivized to fully destroy them rather than leaving even 1 guy alive, you basically force your opponent to either over commit attacks in order to be sure or risk leaving you with active banners. If your opponent over commits, they waste any extra attacks. If you have one larger unit, their decision becomes a lot easier.
Even multiple larger units are easier to handle because of battleshock. Consider 4x5 vs 2x10 or 1x20.
In the 4x5 scenario, your opponent has to allocate attacks to four different targets. They either have to spend their attacks inefficiently to guarantee fully destroying targets OR they spread their attacks out and risk giving you multiple banner activations if they roll poorly.
In the 2x10 scenario they still have to divide attacks, but battleshock makes their jobs easier. Dealing 8 or 9 wounds to a 10 man unit still has a good chance of destroying it. That lets your opponent spread attacks a bit more and be less wasteful, counting on battleshock to partially mitigate the risk of poorer rolls.
In the 1x20 scenario, your opponent just dumps everything at that one unit, and they only need to do 15 wounds for battleshock to do the rest, guaranteed. 13 or 14 wounds will likely get the job done too but not guaranteed.
TL;DR - You want your opponent to either waste attacks or spread out their attacks too much. Smaller units accomplish this much better, and larger units have the additional disadvantage of being vulnerable to battleshock.
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