You have to realize that it isnt every 3.5 seconds of non-casting that you hit, its that a hit goes out every 3.5 seconds and if you arent doing anything during that time it lands. If that was the case no ones auto-attacks would land. If i get a proc, use scathe, flare, or any other action that gives me a split second of downtime, ill be auto-attacking. It's actually a pretty nice dps boost over an entire fight.