The 1st two responses are correct as far as I know. I don't need one and don't like the look of them. I have a friend who uses one. He was playing one night at a bar and a guitarist friend of mine turned to me and said what is that goofy **** their between the pickups? I told him and his response was kind of, "uh, ok".
If you think you need it go for it but most don't. Having said that, depending on the bass I use, I play in different places. On my Spector and my Kubicki, I play pretty much over the bridge pickup. On other basses I play between the pups. I don't think I ever touch the pup covers on any bass though. So, I don't really get it.