This topic comes up a lot on forums so I took some of this post from one my posts on xs650.com.
It's all about weight transfer. When you stop the weight shifts forward to the front wheel and that's why having front brakes is so important.
When the front brakes are strong enough (and tires/road conditions good enough) to lift the rear tire, you are using 100% front braking power. When the rear tire is still on the ground and you are braking heavily, you are probably between 70-100% front braking power.
What's really most important is stopping distances. Sport Rider magazine did a test with a Bandit 600S of stopping distances from 60-0 mph. Front brake only was 151 ft, rear only was 289ft, and both together was 146ft. What that tells us is that when you have both brakes, the rear does very little.. and when you only have the front brake, it takes 3.4% longer to stop than if you had both; and when you only have the rear brake, it takes 97.9% longer to stop.
It's too bad bikes don't look good with only front brakes!
Also, having bigger or better rear brakes aren't going to really help unless your current rear brakes don't even have enough power to lock the rear wheel. Better rear brakes may help with the feel of rear brake so you can prevent from locking it up (which would only increase your stopping distance even more). The only thing that is going to help is better tires, a larger tire contact patch, or more weight on the rear tire. Even that won't help much.
I would also stick with the front brake unless you never plan to never ride it or never ride over 15mph.