The IAS bonus from frenzy only applies to frenzy. Everyone uses bash because bash is a better fury generator (with/without instigation) and a better dps boost (24% in 3 stacks vs 20% in 5 stacks).
I don't think so. Get a few stacks and you'll see your sheet attack speed increasing. If it only applied to frenzy, it wouldn't change the sheet.
If you're looking for attack speed breakpoints, 1,25 unbuffed will let you permaWotB if you can manage your fury correctly, the more, the easier it gets. On the other hand, pure HotA builds don't even need permaWotB, simply pop Insanity, destroy everything and continue running with Sprint:Marathon - in the end you'll be only losing 10% movement speed in comparisson to the double tornado.
Against ubers things are different, frenzy's attack speed buff helps like hell for managing the fury and permaWotB. Once you learn how to time your attacks properly, hitting with frenzy to at least keep the stacks at full, things become extremely easy. On full stacks and WotB with an ordinary Skorn your attack speed already becomes 2.