I just checked wikipedia and there is a paucity of info showing actual effectiveness, and some showing what I just mentioned. It's a little suspicious that there is not overwhelmingly positive data https://en.wikipedia.org/wiki/Anti-lock_braking_system#Effec...
1. If you are too fast to squeeze the brake lever, the tire carcass will not deform properly, giving you the the maximum braking potential. There's a large difference in braking performance just grabbing vs. applying in a controlled manner. You can look up motorcycling racing brake application for more info on this.
2. Breaking too hard can cause a stoppie (a wheelie using your front wheel). Done improperly, that creates a lot of fear in the rider.
3.If you lock the brakes and steer, you are likely to have the front end of the motorcycle dip, causing it to lowside.
ABS helps mitigate all 3 of these issues.