Once you have mastered the roll your fear of capsize disappears and you can venture out in rougher water without worrying about rescues. In fact the better you get at rolling the less you have to do them because your balance and bracing improves as well. Then you find yourself rolling just for the fun of it because you can.
It does take a while to get to the point of rolling instinctively and you have to practice a lot. In fact you should roll every time you get on the water. At first you will find that your roll comes and goes, one day you may think you have it only to find the next time its gone away.
Also you should try to roll on both sides as well, this is harder than it sounds but pays off in spades in rough water and when you are surfing.
Most of all don't give up, with persistence it gets easier and then one day you will find that you are rolling without thinking.
This summer at Optimist we spent a lot of time on our Thursday afternoon sessions teaching rolling and here is a short video showing how its done.