I've found them to be very able actually. The real trick I've found to make using them more strategic is to command them from your drone.
Fly your drone up, tag up the enemies and then you can command your squad to move whilst still flying the drone. This allows you to position them in choke points opposite your position, such as alongside an entrance road.
Then if you miss a shot, or spook the enemies, you can command them to open fire and then get the enemy from two sides. Do remember that if you're undetected, suspicious or hunted status they will not open fire until you command them to do so.
As mentioned in the comments, the Sync Shot, is the real 'killer feature' of the AI squadmates. They are often able to shoot through walls and make impossible shots without line of sight somehow. Perhaps a game glitch.
Another real benefit I find is that they are pretty accurate at shooting enemies whilst in a moving vehicle such as an SUV or helicopter. Especially a Santa Blanca Humvee with a minigun! They have made taking out convoys rather easy, by just commanding them to open fire.
Lastly do remember you can upgrade them! In your skills tree, there is the Squad item, and you can increase their effectiveness here.
- Extra Sync Shot - Increases the number of sync shot targets
- Squad Resilence - A.I. teammates become more resistant to bullet damage
- Born Leader - A.I. teammates become more efficient killers