In my opinon he is just the better VS in ME 3 period. He is more useful in combat than Ash (there is nothing she can do that James and Garrus don't already have covered) and he gets better content outside of combat. Funny thing is I liked her more back in the ME 1 days.
I like Kaidan, but no, he's nowhere near as useful as Ash in combat. Ash, along Liara, is one of the most powerful squadmates in ME3 due to the combo system. Having these two in your team will make the game ridiculously easy, because they can set up and detonate their own combos, essentially making them, to use a phrase RedCaesar97 coined, Shepard replacements.
The problem with Kaidan is that his power cooldowns are way too slow and he has too much overlap with other squadmates, making him very easily replaceable, whilst Ash, due to her unique position as being both Queen of the tech combos(shes sets up and detonates her own fire explosions and tech bursts) and her being the highest weapon damage dealer in the game along with Garrus, will need you to take both James AND Garrus to get anywhere near replacing her. She really is that much better than Kaidan, it's not even a fair comparison to be honest.
On the content I mostly agree due to Kaidan's better Citadel DLC stuff and he has more interactions with the crew, but Ash wasn't the only one who seemed to lack some extra content, the rest of the squadmates did too. The problem is that in the first half of ME3, you were talking on a regular basis with the squad, the VS in the hospital and James, EDI, Liara and Garrus on the Normandy, but after the Citadel Coup, damn near everyone went mute, with your conversations with them turning from proper ones with dialogue wheels, to ME2 Zaeed like conversations. Hell, even Liara shut up, only speaking up to make her presence known during Thessia before promptly shutting up again until you next see her on Earth.
Proper conversations with your squad were ridiculously frontloaded in Mass Effect 3, so front loaded that you can easily forget that some squadmates even exist after the Coup lol. I forgot Liara existed until she came up to my Shepard and said she wanted to go on the Thessia mission, it was like meeting someone I hadn't seen in years. "Liara, is that you? Where the hell have you been?" lol.