You may as well ask "What's the point of pistols or rifles once I have ARs?". Shotguns are best used by mercs with the Hunter/Ranger trait. They get lots of bonuses related to shotguns. Shotguns with buckshot are good for suppression and help poor shooters to actually hit something. Against armored opponents they don't do much damage (unless you use flechette ammo) but even they can be knocked off their feet by the high amount of breath damage a full load of buckshot does. I gave Ira a shotgun (Neostead if I remember correctly) to start with so she can improve her shooting skills.
I wouldn't outfit a whole team with shotguns but it's a fun weapon class. My hunter/medic still uses a SPAS-15 and her kill count is higher than that of some of the AR guys.

Compared to default 1.13 I have the HANDLING_SHOTGUN_MODIFIER set to 0.8 in Item_Settings.ini because I think the handling values for shotguns in Items.xml are a bit high for such basic weapons.

I use SMGs only for my auto weapons people until I get ARs for them or use them as a sidearm for my snipers.

I try to get as many mercs as I can afford, then do the food quest for the rebels so I can recruit Miguel. That already makes 3 mercs that don't cost anything on the long run (IMP, Ira, Miguel). A few cheap AIM mercs like Barry, Meltdown etc. can easily be paid. I usually don't take Drassen directly but try to head for San Mona first to loot some nice stuff in the houses and have a talk with Tony. This usually provides some SMGs, rifles, ARs, ammo and maybe a sniper rifle on the way which is a good start for any offensive on a town like Drassen and the according SAM site. Also the money that Kingpin doesn't need anyway comes in handy too. ;-)