Are you asking about the SmartThings brand motion sensor specifically?
Or z-wave/zigbee motion sensors in general?
SmartThings doesn’t really integrate with the ADT pulse system, but that doesn’t mean motion sensors are useless.
I use motion sensors to turn lights on/off, probably one of the more common uses, as you’ve noted.
Same thing for ceiling fans, too.
I can get alerts if there’s motion anywhere when I’m not home, presumably your ADT pulse system serves a similar purpose.
But i also get alerts if there’s motion in the kids’ room when I am home but it’s after their bedtime. That’s something I’d like to know about, but doesn’t quite rise to the level of a call from the alarm company monitoring station and a police dispatch.
If I had an elderly or infirm family member, I could use lack of motion as an early warning that something’s wrong, e.g. grandma always wakes up at 6 and goes down to the kitchen, but she didn’t this morning. Maybe someone should go check on her in case she fell.
Motion could be used to trigger all kinds of voice announcements using connected speakers, although I don’t have any personal examples of this.
So if you already have a home alarm system with motion detectors, and you’re only interested in detecting motion for home security purposes, then sure, SmartThings motion sensors are kinda useless.
But the other examples I gave are just off the top of my head. You’re limited only by your imagination.