Under 'traditional' foot-to-pedal braking, and as far as turn signals go, I view the icon lighting up as just a 'cute' addition. BUT, I think that icon lighting up plays an important role with regenerative braking. I almost always have my regen mode set on high, so I use my brake pedal once in a blue moon. As aggressively as the car will brake under that setting, I've always been a bit concerned about whether or not the brake lights are properly alerting the following traffic of my braking, and this at least gives me some indication that it is. I hope/trust the brake lights (and icon) are properly synchronized.
I view it as something to check/notice from time to time, but not something to actively focus on while driving.
BTW, I wonder if the icon will show if one of the bulbs is burned out, or otherwise non-functioning. That would be a nice plus.