Our advice remains: Never sign an attrition clause. However, you must implement this as a strategic and not a tactical decision. Thus, you use this strategy at the very beginning, for example, when doing site selection. Means you will not be able to be in Las Vegas in peak season but maybe Miami in August. You have to judge whether this will hurt your bottomline more than not having an attrition clause.
Another approach is not to take a room block at all. Many of your attendees "go around the block" anyway, so why fight it? If this is not feasible, then at least severely limit the number of official hotels and then "encourage" all attendees/exhibitors to use them. For example:
really promote the advantage of staying in these hotels (walking distance, VIPs stay there, etc.)
offer a show registration discount for those who stay in these hotels
offer additional priority points to those exhibitors who use these hotels for their staff
offer merchandise coupons, restaurant discounts, etc. (provided by the hotel) to those who stay in these hotels
Note we predict in the long term, show organizers will actually get out of the housing business and let the attendees fend for themselves.