Regarding the "to fight blobbing" postings - I do think that auto-withdrawl is the better option there. The problem is not that the larger army does more damage (it should), but that it wipes out the defenders. That leads to people always blobbing around because being hit by the enemy blob in small units is suicide.
Regarding the disproportionate damage - I agree on that as a purpose. It has to come at a price, though, otherwise people will use it all the time.
One thing has not been mentioned: To engage the enemy archers in melee combat without having to cut down all their melee troops first. This is also the idea of flanking, to a degree.
I suggest we merge these two ideas. I don't think a real ambush mechanic will work in BM. We don't simulate army movement in enough detail right now for it to work nicely, and adding that would be a lot of work.
But how about setting up "flanking maneuver" as a combat option, maybe a deployment setting? Give it a chance of discovery. If you are discovered, your unit starts at the very back of your own army deployment zone, as it has to maneuver back to its own army. If you succeed, you start behind the ENEMY deployment zone, and will probably hit their archers in the back within 2 turns.
Fairly easy to implement, allows a small force to do disproportionate damage (melee troops engaging archers are usually a slaughter), adds a risk factor, and with a bit of strategy (one bait unit deployed regularily, two flanking units) you have a nice trap for the enemy blob.
