The speed at which things are killed on any given difficult are irrelevant to the story.
That's why I listed it under gameplay.
He let him talk because the Illusive Man wanted to talk to him. Which is the story element in it. Giving the player yet another chance to try and stop him from his current path that fails.
Because you have to beat him to progress game play wise. Story wise how ever he was able to hold his own against Shepard though not very well. Hence the calling in of the air support. Realistically the fight would end after that first airship is called in. How ever game play reasons that would be extremely boring to have happen. So you get him recharging his shield a couple of times and hiding from air ship a couple of times.
There was literally zero reasons to let him talk aside from "plot needs to progress somehow". And you're mixing the gameplay into story and vice-versa. The whole episode was handled poorly. There's bunch of ways they could have handled this in a way that made more sense for gameplay and for story.
1) Make Kai virtually unbeatable opponent, when your medi-gel supply runs dry and health-bar hits certain(low) point cue the cutscene where he beats your ass and takes the catalyst.
2) Cerberus has small army, you're running out of ammo. You have to run or they'll slaughter you. Cue the cutscene where Garrus and James have to drag you away from combat.
3) Kai isn't tough, he's sneaky, while you were distracted mowing down countless hordes of Cerberus mooks he snuck behind you and stole the catalyst. Cue the cutscene of him standing on the altar, gloating as gunship daringly wades into the temple spiriting the sneaky bastard away, and shooting the pillars on the way out, thus trapping you.
These three I came up with off the top of my head, in 10 minutes or so. Writers had 2 years, you do the math.