I am going to leave mine on. I have the spare mounted bike rack and it has several advantages as I see it:
1) It protects the tire rubber from the SoCal sun
2) It protects the tire from the bike rack rubbing on it and leaving wear marks
3) it hides the fact that you can just unbolt the tire and walk away with the tire, bike rack, and bike!

With the cover on you see the bike rack padlocks and it appears that the whole thing is locked on by this.
I have some locking lugs to put on this weekend to help that last one! I also had to cut a small square out of the center of the tire cover so I could leave it this way. You can see pictures of it on my site.