I have only known of one person who has had trouble with a speed sensor, but it was an issue that was hard to diagnose and adversely affected the way the van ran. With that said, when I saw these simple guards on Agile's website I decided to try them out.
Here is a picture of the front wheel speed sensor. You be the judge of how exposed it is:
The guard couldn't be much simpler to install. All that is required is to remove a bolt and install a new one with a bit of Loctite Blue on it.
Simple, and hopefully effective. Time will tell.