The rear brake is fairly worthless.
I can make a good argument against that point of view.
I learned how to use the rear brake in my dirt biking days where using the rear brake to slide the back end around is a vital skill. In street bikes, the same, but less aggressive, technique is normally called trail braking.
There have been reports in magazines where using the front and back brakes together reduces stopping distance by about 5 to 10 feet. I'll take that any day, and there have been some days when I wish I had it.
Next, using the rear brake can actually reduce fork dive and reduce risk of locking up the front wheel by taking some of the braking force off the front end.
If you ever have had the front brakes fail, you had better know where the rear brake is. Yea, I've been there, done that. But if you never use the rear brake, it is not likely you will find it in an emergency.
Most guys who don't know how to use the rear brake say they don't want the rear to slide out. Using the rear brake only when going straight will fix that problem. If you can't use the rear brake without sliding out the rear, you shouldn't be riding a motorcycle.
Other guys say the rear brake is too sensitive and locks up. My response to that is fix it so it doesn't. Too much air pressure in the rear tire can cause a small contact patch and cause easy rear wheel lock up. Try to adjust the brake pedal down a bit. Try a different pad compound. On dirt bikes, I have cut off some brake pad area to get a better feel on overly sensitive brakes.