The way I see it, everyone has missed the point so far.
The benefit of it being expressed as a factor (1.3) of the stall speed is so that approach speeds are proportional.
Take the PA28 for ex. Stall is say 49kts (from memory) and approach is circa 65kts. Thats a 16kt margin over stall.
A jet however that stalls at 110kts, should approach at 143kts (or 1.15 stall over threshold) giving a margin of 33kts. WOuld you feel safe approaching a jet 16kts above stall? I wouldn;t. Its all proprtional.
I realise I may not have expressed myself particularly well here and welcome all abuse as deemed necessary
