I think the use case for the ADP/ST alarm panel is hard for many folks to wrap their mind around, for me it was a perfect fit. The ADT/ST panel is a commercial 2GIG GC3 alarm panel with the integration of the ST hub capabilities. Think of the alarm panel and ST hub as separate systems that talk to each other
The reason the panel requires specific (900mhz) sensors to trigger alarm events is for reliability and integration with the alarm panel not the ST hub. There are many threads of general HA sensors providing unexpected results, you are very unlikely to see this with the ADT sensors talking to the alarm panel. Depending on your security needs, the reliability of ST and general HA sensors may be fine. For a focused security stance, the ADT panel provides a more reliable system that can also communicate with ST.
The ability to activate monitoring monthly as needed is also beneficial to some. When we are travelling I’m likely to have monitoring turned on. When I’m local I will self monitor, all the alerts still go to my phone either way.