Well, worm-beings are all pretty much slow anyway, their advantage would probably be stamina and weight capacity.
One hopes the right of ownership for this on-land beings will have no bounds, as it happens with the fliers... And having to buy equipment for these beings to be able to actually carry things would only bring more realism. Hmm, now that I think of it, the worms could bring another kind of knightship, worm knights! Or if the worms are big enough, they could become a major way of transportation... oh well, that'd be wierd.