A belt shortening clip does something different than a locking clip.
A locking clip is just a place holder to keep the latchplate from sliding along the webbing during normal driving. Think of it like the pin in a belt buckle going through the hole in the belt. Pin doesn't have to be that strong really, because the whole belt is holding all around your body, even when you bend over and put pressure on the belt.
A belt shortening clip actually makes a loop in the webbing out of play. Like when you can't find a belt and use string to tie two of your belt loops together. Your pants still stay up, but it's because you are bypassing the too-big waistband altogether. The string has to be pretty strong though, or when you bend over it's going to snap and release all the extra material and leave you hanging in the wind.
The problem here in the CPS world is that locking clips and belt shortening clips look alike, so it's important to make sure that you use the right one for belt shortening. You can use the wrong one as a locking clip, no problem. Honestly, you could use a chip clip for a locking clip and it would be plenty strong enough because the seat belt is the weight bearing component of the system.
Locking clips are made out of a nickel cadmium alloy that is actually quite flexible. It will bend in a crash, possibly enough to pop off the belt. (I have a bent post-crash LC somewhere around here).
BSCs are made out of hardened steel. It doesn't bend unless molten and can withstand crash forces.
Here's a pictorial on how to use a BSC (Lots of older GM vehicles still running around my family, so I've had more than my share of practice)
http://www.car-seat.org/album.php?albumid=75