Slowing classes are generally in groups for other reasons, not slowing. Since slowing is a basic element in the game to the point where balance is designed round it being present, it is not unreasonable to have lesser versions of these spells distributed to other classes in some form or another. Slowing classes still get better versions of the spell. The only time having this belt proc is useful is when there are a ton of adds in the group, but under those circumstances slowing classes are better utilized for things like CC/dps/healing anyway, not slowing. The belt procs help avoid a wipe that might otherwise happen.