Horrible Heavy Hitters?

Do heavy cavalry still have an important role to play in Warhammer Fantasy? For many people the answer is no, but Lange Grinevitch has always encouraged listeners of the jadedgamercast.com to continue to use the heavy hitters in 8th edition. He suggests that cavalry is no longer a blunt instrument, but when used effectively can change the course of any game. I agree with Lange, and think those that have totally excluded heavy hitters, like my Choas Knights, are missing out. Fellow RHQ blogger Sean Davis emphasis the importance of having a clear role for cavalry when putting together your list, and this I believe is the key to using cavalry in 8th ed, but what roles should cavalry fulfill?