Dex penalty definitely works against player, afaik it seems to work against enemies (anecdotally, if someone had more thorough test then hopefully they will post).
Took a look at my rogue last night when he was stunned, screen showed 51 defense, which is base rogue defense and +1 from studded leather armor set. Base warrior defense is 45, and for mages it is 40. Loghain runs Rally so he should be getting +10 from that, and then player may be suffering from -10 attack from War Cry.
I would agree that low attack can be problematic against Loghain. FWIW I loaded up a save I had w/ Leliana (level 20) transitioned to cunning dagger rogue (transitioned not respec'd, so still talents wasted in archery) and she had 114 attack, and that was enough to auto-attack to victory w/ paralyze runes and concentrated deathroot. Defense was 92 with wade's boots and 98 with Bard's although in this particular test it didn't matter. If she goes with bad gear the fight does get progressively more difficult, especially because she did not have enough talent points for Riposte. Warden should be in better shape with more attribute points, and potentially tomes for points.
Point is mainly just that as you move away from any "min-maxed" build it will get harder, or if you aren't running end game gear it gets harder still (obvious).
I agree that if dex tank, should run dagger. I build Alistair this way sometimes, but I don't usually make him solo anything.
-This evidence is irrefutable.