The slightly bizarre thing with slings is that it is perfectly 'acceptable' to use a 2-point sling which you can change the length of or even move up and down the accessory rail, wrap it around your bicep and forearm to gain stability (just as in the US Marine Corp manual :-) ) but you can't do the same thing with a single point sling used in the normal way (i.e. hook on to a handstop which can be moved back and forth along the rail) which is to say - detach from the handstop between lanes.
Given the variety of target inclinations (up or down), etc, etc the 2min per lane limits just how much you could change the fix-points anyway.
Banning slings altogether has been tried on BFTA AGMs but fails. The prejudice against single point slings leaves the double point sling in place but it does, without doubt, give an advantage over no-sling if someone knows how to use it and has practiced such a technique.
Slings or no slings - it is not based on logic, just a bonkers discussion and "gut" feeling that one is bad, two is good. Maybe it is better to have all slings disconnected from the rifle other than for carrying them over the shoulder i.e. as in the "hunting" mode :-)
BFTA Secretary 2012-2014
CSFTA Secretary 2014-2016