Have the inserts, like them for a couple reasons:
1. can adjust for windage (in terms of rail, mount, screw holes being slightly off) and elevation;
2. if have to adjust eye relief (add recoil pad, change shooting position), scope slides through inserts thus windage & elevation adj. remain set (though cant can be off); and,
3. not mar scope body.
I took the rings off the weaver base, but rings back on, then moved scope in the rings, and then shot at 300 yards - was on paper.
Have heard cons about one piece bases too - stress receiver if holes not perfectly aligned and also reduce access to ejection port/mag.