Mudflaps can make a huge difference to your winter group cycle rides. Most mudguards finish half way down the back wheel, which means you stay dry but spray still hits the rider behind. Adding a mudflap catches that spray and everyone gets home (relatively) clean. All that expensive winter kit doesn't need washing each time, therefore extending its life. More importantly, you don't have a face covered in road dirt, which can only be a good thing. Once everyone in your group has a mudflap, you'll hate anyone who turns up without one.
As you're protecting those behind, you may as well protect yourself and your bike too. The front flap can greatly reduce the amount of mud and spray hitting your feet and the bottom bracket. When fitted as pair, the mudflaps provide a stylish finishing touch to your bike.