What? There is no BiS follower weapon. Depending on build and class, a lot of people have more issues with the Grift clear than the RG. Rathma Necro? RG is easy, clearing the trash isn't really that hard either, but it's much more dangerous than the RG. However, I will point out that RG kills at 90 average at about 10-15 seconds. It takes 8-10 minutes to clear to the RG though. So where does the importance lie?
I'll also point out that there are different builds for followers, like The Tormentor and Overwhelming Desire vs. Sultan of the Blinding Sand and Ess of Johan for the Enchantress. There's even a RG killing Templar build that focuses more on Bul-Katho's Wedding Band and elite damage affix. Saying a chance to freeze at 20% isn't always "the best" option would be an understatement.