Realistic: No
Annoying because of overuse: Yes
Should people simply pay attention to who is around: Yes
Now for the reality.
1) People often DON'T pay close enough attention.
2) Getting wrecked out of a race is much much more annoying than a horn happy driver.
3) People often use the "didn't know you were there" excuse to justify deliberately wrecking another racer.
There are aspects of sims that make them inherently unrealistic. These aspects can be used to improve the experience or to worsen the experience.
The cars are free and fully repaired simply by hitting shift+s. How realistic is that? Its not. At all. And the result is little respect for in game property (ie, the car).
So, for lack of common courtesy and respect, for lack of driver awareness, and for the existence of human pettiness, I suggest putting a horn back onto the Formula cars.