Good points guys.
I figure I could loop the straps around the top crossbeams of my squat rack and potentially use band pegs to keep it from moving, not that I'd imagine the sling will change position at the top too much when the slings have enough weight put on them.
If I'm interpreting correctly, you guys are suggesting the bar could swing because the bench pressing movement isn't vertical? I would've thought that it wouldn't be too hard to control the bar's horizontal movement, even during failure, as controlling the bar's horizontal movement as it descends doesn't require me to actually lift the weight on the bar, rather just guide it down to the stop point.
I figure having the straps stop at chest height would make the most sense. Perhaps I'll find more difficulties when I get them and test it. In any case; I'll have a spotter and metal safeties (albeit dented ones) in whilst I'm testing the slings for benching. As I said, 4 straps only came to $50 odd, so they should be useful for squatting in the least.
TBH i never use safties when benching, I just don't use collars and let the plates slide off the bar when I fail.