I’m just a very-satisfied user, not part of the dev team, but I’ll jump in. I don’t think it matters if your system is monitored or not - your choice, but the benefits of ST integration apply either way.
You can use your alarm system sensors as sensor inputs to ST. However, the big limitation is that ST only knows about zone stati, not individual contacts, etc. So, for me, the alarm->ST communication is of minimal benefit. But you could definitely use your existing sensors for ST if the granularity of your zone implementation was fine enough.
But, don’t underestimate the value of ST controlling the panel - I rarely touch my panels anymore because ST automatically arms/disarms 90+% of the time. And the remote capability of ST telling me the status (normal or intrusion) and allowing remote control is very useful.
To repeat, I think all these features would be worth the integration whether it not I had a monitoring service.
EDIT: Just to clarify, I used this project by @d8adrvn and @ritchierich :
AD2SmartThings