doing the most damage or being the closest has no effect whatsoever on what the computer does.
That seems a fair assessment to me, as well, though unlike your experience, I find that the computer AI *loves* killing my summoned creatures. If you keep the summons closer to you than to the enemy, they will provide a fairly large wedge of cover for your back-rank folks; this way the archers and mages, at least, while trying to kill your squishy folks, will often hit your summons instead.
Probably the nicest thing, in my opinion, about the man-at-arms tree with regards to battlefield control is that once you have attacks of opportunity and a weapon with a good knockdown/blind/stun chance, things will waste AP to run around you, only to be knocked out of the fight. Then you can use *your* AP to position yourself for your next attack of opportunity - and with charge, sweeping attacks, and ranged knockdown abilities, you'll often not have to spend many of those AP on anything but attacks.
Of course, if you're only asking about that one fight for which you've posted the screenshot, the ranged comments are useless to you ^_^
