It looks like Swen has indicated in a reply on his blog post some of the ways in which the multiplayer experience complements the single player one (rather than substitutes away from it) - at least in w.r.t. dialog:
so the idea is that in single player you can select a character type for the AI in which case the AI will answer in line with his character. Alternatively, you can opt to always have it your way, in which case the AI will not interfere. It's really up to you personal preference, though putting the AI on random will give you the most fun imho.
By giving coop players choices to disagree with one another, a framework (involving charisma rolls etc) would seem to be in place that puts the infinity engine party quarreling to shame - being much more dynamic and responsive (to your chosen character's statistical profile, it would seem) than the more solidly scripted party incompatibilities in BG. I wonder how many personalities this will cover? Can all 4 party members have very distinct personalities?
Anyway, the fact that they are giving the option to prevent AI from interfering is also a promising sign, since there are obviously some people who like to 'get their own way' all the time. Personally I think this sort of customisation and innovation should be encouraged. Should give game journalists more interesting things to write about (if they weren't so obsessed with their 'visceral combat' and 'mature themes'.