Like I said, they're unnecessary if you're a safe driver. There are an awful lot of drivers playing on their phone, playing with their touchscreen in the car, drunk, incompetent, or distracted.
Well to put a sharper point on it -
1) judging who is a "safe driver" is much easier said and done, and fraught with bias (the overwhelming majority of people rate themselves as 'above average drivers').
perhaps even more important is...
2) even
if you are a great driver, you are constantly surrounded by below average drivers. Those features react to all the dumb, unpredictable moves that drunk/distracted/horrible drivers make.
Blind spot warnings and backup cameras were a lot less useful when vehicles were designed so that you could easily see out the back and rear of them. My mom's corolla has a back up camera . . . and without it it's almost impossible to see well enough to reverse into a spot because of the small rear and back side windows. This is a significant safety downgrade from the older corolla.
As we've covered in previous threads, the decreasing visibility in modern cars is largely attributed to the much more stringent safety standards they must now meet for impacts. It's created a sort of design paradox, where the passengers are much more likely to survive an accident without serious injury, but the structural requirements lead to designs which have greater blind spots and are therefore more likely to cause accidents (absent all the new crash avoidance systems).
Getting back to #2 above - you are constantly crossing paths with terrible drivers. I personally would rather be in a modern vehicle with less visibility than one with smaller blindspots but with terrible safety scores for side/rear/roll-over accidents.
tl;dr - it's not about you as a driver. It's about driving as a whole.
bonus: Driving could
also be made a whole lot safer if we made it a heck of a lot harder to get and maintain a license.