That is why you need to work on a bomber brace and many different bracing techniques. That way when someone sneaks up on you they will roll when trying to roll you because your bracing skills will be stronger than their messing around skills.
Yeah, you should work on it. However, as wavehog, mark, and illusion have clearly pointed out, sometimes it's the water itself that sneaks up on you. Buying into the myth of the "bomber roll" or "bomber brace" can come back and bite you in the rear. Among other things, I've snapped two paddles bracing, and also had the boat stripped off my body on one occasion.