These are nice to have on your bars to keep water and other debris out of your rails. That being said you do not need to buy these if you are buying the railgrabber also because a set of four are included. I learned the hard way. No biggie I just payed like $8 more than I needed to. Just read about the product or ask a rep before purchasing both.
Unless you use some other accessory at the end of the bars you need these. You can live without them, but to have a yakima rack system and unfinished bar ends would be silly. Without these the ends are rough and ugly. And without these you will probably get some stuff and moisture inside the bars. You can live without them if you like but it would be silly.
I agree with everyone in the fact that we are writing a review about caps. I lost a cap a while ago but then decided that i had gone capless long enough, so thanks to the peps at MOOSEJAW I am no longer driving capless. and i have 3 extra caps in case my truck decides to go capless again.