The reason for this is that, in my New Zealand cpl flight test pass, I was marked down for slipping on final. The problem is that if you were to get into an out-of-balance stall (just say) at the low altitudes of final approach, you will not recover. If you are flying in-balance s-turns and for some reason you stall, you may be able to recover.
I've been similarly criticised for this flying with UK instructors when sideslipping a PFL.
Personally, I think it's cobblers. A low level stall will almost certainly kill you whatever sideslip you have on in all likelihood, but so long as airspeed as maintained at or above 1.3Vs and below Va the risk of stalling is no greater than in a normal approach. At the same time, a sideslipped approach is much better for maintaining runway centreline and touchdown point visibility than an S-turn. That and the bank angle of the S-turn increases stall speed, so you either have to increase speed or reduce safety margins. Additionally, a slowly approached stall with full sideslip is unlikely to actually turn into a spin, that usually comes with more rapid deceleration.
However, we will fly the way that makes our examiners happy of-course
Better still, if too high, take flaps early. Won't work on all aeroplanes, but on many it will.
G