I let Loghain live in my last playthrough for the first time, and I'd definitely recommend worth trying it at least once, to the skeptics.
To be honest, while I was playing the game, I didn't entirely see why I should hate Loghain so much. Ostagar seemed defensible - if a little shocking, at first. Cailan struck me straight away as someone who had no idea what he was doing - I kind of predicted we'd lose the battle, it seemed the most obvious way it'd end. What surprised me was that Loghain retreated not when he saw the lack of signal, but when it appeared. After reading the codexes more thoroughly, and finding out about the extent of Loghain's Orlais hatred (which is hardly without justification), and given how political the Grey Wardens actually are in practice, I'm not surprised he turned down extra reinforcements and I'm not surprised he made the decision to retreat. What irritated me more was how last minute it was - if I were in Loghain's shoes I'd point blank refuse to risk my soldiers on something so foolish - but I suppose nobody but the Grey Wardens predicted the bulk of the hoarde, and Loghain's skepticism lead him to downplaying their advice, good or bad. To his army's near-peril.
So no, I don't think Loghain killed Cailan, or Duncan. He decided taking a small hit was better than a large one - because, y'know, you'd rather lose one battle than lose an army and a war. To be honest, I question whether the army gathered would have been able to take the forces, or be completely overwhelmed - personally, I think they stood little chance. And so if Loghain hadn't withdrawn, I'd question at all whether Alistair and the Warden, even if saved by Flemeth, could gather an army large enough to allow them to tackle the Archdemon.
I guess what threw me more was the outlawing of the Wardens - unless Loghain somehow saw *them* as responsible for Cailan's death, or whether he was taking a ruthless, no-loose-ends approach which meant there'd be as little political opposition to his new leadership (which totally backfired). From a tactical point, I can sort of justify it but it's dead ****** cold. Asides from that, I think a lot of the dodgier tactical choices were made because he had a lack of opposition and debate amongst his advisors with Howe and himself - I reckon if he'd had someone more compassionate supporting him, maybe it wouldn't have been such a disaster. A lot of the nastier outcomes were part of Howe's influence.
So yeah, even on my first playthough (as an elf, no less), I nearly saved Loghain for that reason - I can see where he's coming from, and thus would have thrown him to the Wardens first time if Alistair hadn't spoken up (more because I didn't wanna lose his gear, than any RP related reason). As Anora says, the Joining can be fatal (have no idea how she knows that, but oh well), so it may be a death sentence for him. If not, he might be useful. Given that he was a former war hero, I reckoned that he wasn't completely without principles, and probably could perform some kind of redemption.
I suppose I'd agree with the criticisms saying that the game doesn't expose the complexity of the situation very well. Like, we're told certain things at the beginning (Wardens are good, being a warden is an honour, treated by respect by most - or should be etc. etc.) which are at best questionable, if not complete horsepiss. That's fine - I think starting from a point of unreliable narration and bias with Duncan and Alistair is actually really interesting from a literary standpoint, but they don't challenge that view enough, I think, in the game, or make it clear that they're *unreliable*. I mean, I personally really like Alistair's reaction at the Landsmeet because to me it shows off his worst flaws - his petulance, how *subtle* and rarely shown his vengeful, nasty streak is, and also how he desperately tries to maintain this romantic image the Wardens like to advertise, at the cost of weakening their forces against the Blight (3 ferelden wardens are better than 2). But plenty of people just think his reaction is completely justified and don't see the clash of characters, principles, and commitments going on.
It's annoying - DA:O has some fantastically nuanced characters, but it doesn't expose them very well and doesn't allow them to object or question what you do.
Edit:
Also, I'll add - I really, *really* liked Loghain as a companion. He was very forthright about the situation he was in, but in person (and not in DRAMATIC LANDSMEET HYPERBOLE MODE) he's very funny, and his candid demeanour does a lot for him. Warmed to him very quickly. Love his voice as well, it's full of character. I didn't expect to be a Loghain fan at all, but there you are.
Does he turn up in DA:I? I've played through II but I'm not getting DA:I until after my final exams. If he turns up for more than a cameo, I'll get it.