I think whether a gun has interceptors isn't necessarily a sidelock vs boxlock thing although the sear engagement of sidelocks being more crisp and able to be made light would indicate an interreceptor as a real plus. I think from my readings the gunmakers of boxlocks felt the interceptor wasn't needed.
Now as someone that has witnessed first hand an accident with a boxlock falling and discharging, I can say having an interceptor is a great thing.

This ain't a dress rehearsal , Don't Let the Old Man IN