The reflective sidewalls I've seen are quite visible at any angle where you can see the sides of the tires. So, except for directly behind the bike, the reflective sidewalls are very effective. Because reflective sidewalls use microspheres, they are effective at any angle. the plastic reflectors rely on small corner reflectors which have a limited effective incident angle.

Although reflective sidewalls are unlikely to prevent a T-bone type of crash, consider when you make a left turn you will show your broadside to approaching cars that will eventually have to pass you. Seeing the very recognizable image of two bike wheels makes it easier for at least the first car or two to know what that blinking light ahead is.