At best glide in a C172 (~65kts) with the full 40 degrees of flap you have a stonking descent profile! Approaches that are still high in this configuration should be abandoned and flown again properly. There's never much need to side slip a C172 IMHO.
I have flown a C172 with full flaps and 45 bank with a ROD of less than 300 fpm... This was due to heavy thermals in the area.
And in an emergency engine failure situation where you are about to overshoot it is good to know what will happen if you try sideslipping. The alternative might be hitting something really hard.
So practising sideslip with full flaps (at a safe altitude) is not a bad idea at all.
(geewhizz->the latter not aimed at you, as this was you point exactly ;-) )