Ashley has Inferno Grenades. That in itself is ownage. Kaiden has what: Barrier? OOOOOOOOOOOOOOH!!!
Grenades are limited, you know that right? Reave on the other hand is limitless and if Kaidan wears his cooldown reduction costume it has around 8 seconds cooldown. It can be made to affect enemies in an area making it one of the perfect primes for explosions. Not to mention it provides a damage reduction buff for Kaidan which lasts for 9 seconds, making it permanently last on him.
And its because of Barrier that Kaidan is second tankiest squad mate out there, after James of course. Enabling him to fully use assault rifle and be out of cover when he is shooting and not dying in process. Ashley and Garrus both have this weakness therefore its more optimal to use sniper rifle with them so they keep their distance and take cover a lot.
And lastly Kaidan provides best versatility when it comes to breaking barriers/shields and weakening armor and he provides enormous power damage bonuses to squad mates, making him best squad mate for Vanguards, Adepts, Sentinels and Engineers or any squad that is focused on power usage and not weapons.