Well - they break ice. And touch bottom before your boat does, increasing the drag you have. And make it a b!tch to break suction or slide your boat around.
And they make your boat turn sharper than a Honda Civic. Probably their most useable feature.
Boats break ice just fine without them, I wish mine didn't have them, and I have an 18' boat that will have them cut off before I am done. Buddy has a Prodrive boat that he drives to work every day, he breaks ice all the time just fine. It may tend to slide up on the ice a little more than mine when it is several inches thick, but I don't think it's worth all the other disadvantages. Unless you live in Antarctica by chance.