Is there any reason to target the defensive riders?

There is no reason to target defensive riders specifically in terms of reward like there is for money or health riders. However, there may still be reasons to take them out:

• they're in the way of you killing someone specific, e.g. there's a money rider next to them and they'll block your attacks
• you're being attacked by a group and don't have nitro to boost out
• every kill gives you money and experience, including of defensive riders

I'd also like to add that there's another way to kill them: block one of their attacks and strike back, they can't block your counterattack. Still, if you meet one alone on the road like in your screenshot, it's fine to just avoid them.