Is there any way to make the ai use their secondary weapons? It seems like there would be no point of them having secondary weapons if they can't use them.

I'm almost sure they never use them, it's a filler slot.

For SWAT 3:

They actually use their secondary weapon, if they run out of ammunition for their primary gun and, as far as I was able to see it, do not have time to reload, as they still face a suspect at the same time, while they are running out of ammunition. Give your team members the shotgun and have a huge amount of bad guys on the map and you are likely to see them using their pistols.

The best way to see that is to play an offline LAN game on the first map, you know, the two-level house, play as a terrorist, fill the map with 30 AI controlled bad guys and give the AI controlled SWAT team shotguns and I guarantee you that they will use their pistols quickly, no matter if it's the Colt 1911 or the silenced pistol.

For SWAT 4:

It's true here, I have never seen the team members using their secondary weapon, EXCEPT if you give them the grenade launcher, because they are never using this gun. So, if you want them to use their secondary weapons, give them the grenade launcher of the Stetchkov expansion and they have their secondary weaponry in their hands as soon as you start the mission. I do not know why they cannot use the grenade launcher at all, but, you know, the AI controlled SWAT 4 officers are truly dumb in many ways and the reason why I keep playing SWAT 3 anyway.