they could be used on isolation movements but would act against you when you do deadlifts or cleans.. I get the theory that thickening the bar will strengthen the forearms which is what the poster above said
We have a bar at my gym that is about 3 times bigger around than the Olympic bar and we use it every other week and it definitely works the forearms well while doing curls.
I have several pairs. I love them and use them for many lifts. I think that they are great if you are training for any grappling combat sports. Wrestling, BJJ, Sambo, etc.
If you are in need of grip work they are great if not then they just reduce the amount of weight you can use. Get em if you want an extra tool to use but they aren't necessary. You can still train the grip and forearms the old fashioned way with a barbell or dB's. Wrist curls and static holds at the top of the DL are highly effective. But like I said if you need a little variety then why not.