User blog comment:Emptylord/Archived/New Boots and Enchantments (2015)/@comment-24795478-20150515124745/@comment-3974211-20150516234748

You get 4% from masteries. An item you pay for only granting double a mastery is... no. Too weak.

Furor only works in combat, while Flutter works all the time. Flutter is designed for supports and tanks, primarily, Alistars, Sorakas, Sonas, etc, (and Janna, if she wasn't already).