The rear brake is great, I use it all the time. Yes you will stop faster with it. I understand that some of the older generation speedy's are having issues with their rear brakes... I dunno the problem, but if you can lock it there's not a problem (maybe too strong even) I adjusted my rear brake lever all the way down ( towards the ground ) so that it would take that much more effort to lock it. A good rule of thumb is if you can fell yourself pushing it you're pushing too hard. Rest your foot on the lever and let teh weight transfer apply all the pressure to the lever. When you lock it, you lose control and the gyroscopic effect that the turning wheel gives you.
I use mine in the turns a little, honestly it's a bad habit that i'm trying to fix. If you reallly think your brakes useless, try this. Next time you approach a stop sign or red light, apply the rear brake before the first, just slight pressure, your not trying to stop with the rear, then stop as you normally would with the front. You'll find that by applying the rear brake, you've bound the rear suspension a bit, the weight transfer to the forks will be much more controlled and not as drastic.
The best braking advice given to me was in Motors school. You brake just like you grab a boob, progressivly. If you reach up and grab it you're gonna get slapped, but if you squezze it lightly and progressivly sqeeze harder you'll be hearing a moan (girl or tires).