They can't hurt. The angle at which they're most effective puts cars in a path heading straight at you. Well, if they're moving, that is...

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.

I have reflective sidewalls, but I think they help very little -- the sidewalls were not a factor in my decision to purchase the tires. Unless you only ride in dry conditions, the reflective part will be so covered with grime in no time that it will be virtually invisible.

If you want to be visible, wear proper reflective gear and carry decent lights.