On most characters, no, not for my primary abilities. I use them for interrupts, triggering trinkets or procs, emergency cooldowns, and other utility. I have small hands so modifiers don't work well for me and there are finite keys I can reach without having to move my hand a lot. If I can't comfortably reach a keybind I risk not being able to hit it fast enough when I need to. The side button mouse have been a lifesaver for that kind of stuff. But my main abilities that I'm hitting constantly are still usually the number keys on my keyboard.
Healer is the exception, but it's a big one since my Horde main is heals. On my healers I use every single side button, some with modifiers even so I can double up, for everything in my toolkit. On a raid night I might never hit a keyboard key in combat.