Bobbins: In my opinion, the foam disk is an amazing tool. It is cleverly designed for ease of use. The combination of the dense foam and the slots, nicks, or slits do a fantastic job of keep your braiding cords under appropriate tension. For this reason, in most cases, I just use my plastic bobbins to prevent my cords from tangling. If the slots hold the cords
under tension, there is no real reason to use weighted bobbins. 

Weighted Bobbins: If you are braiding with a finer diameter cord, like C-Lon Beadcord combined with beads, and your cords are slipping, then the weighted bobbins are helpful. 

Counter weights: The term "counter weight" refers to the weight that that can be attached to the tail of the braid. It is called the counter weight because when one braids on the Marudai, the weight attached to the braiding tail "counters" the weight of the wooden tama (heavy bobbins). With a foam disk, since the cords are held under tension in the foam slots, it is not as necessary to braid with a counter weight. If I use a counter weight, with the foam disk, its main function is to steady my braid. I use a small hex nut and clamp (42 grams combined and 50 cents each from Home Depot) & attach the hexnut and clamp to the braid tail. I do find that using a counter weight, when using the foam disk, does help even more when braiding with C-Lon and Beads.

Considerations: The quality of your braid can be influenced by the weights you use. If you find that your braid is slack, whether it is a fiber only braid or a braid with beads, it is probably because you are using to much counter weight (weight on braid tail). Check your braid frequently and make sure you like the way it looks and feels. Adjust the weight of the counter weight to get the end result you want.