Throwing knife makers who do affix handles use either a really thick piece of solid leather, or a vulcanized cardboard like material that makes for a harder, yet durable handle with less friction than leather. Both materials tend to be affixed with large rivets that are pounded down into wide washers to spread the area over which the handle is gripped by the mounts. Even so, these can come loose once in a great while, or, in the case of the vulcanized material, crack. Handless throwers are perhaps more common in the professional arena. Also, some hands seem to fit a handled knife better.
Some throwers feel it improves release, or makes the knife less uncomfortable to hold.
In the summer of 1996, Bob Karp taught me a way to make inexpensive, functional, and easy to replace handles on otherwise handleless throwers. The technique is described below. Start with a strip of linen about 1 or so inches wide. An old sheet, or even a tee shirt may do just fine. Wrap this linen up to three times around the knife handle letting each turn overlap the one under it a bit.