Being a new (but now frequent) guest to London TMA, I have initially found this "clearance limit" stuff rather surprising. Never encountered mandatory hold requirement elsewhere and hadn't I been given a warning by my more experienced colleagues, hold would the last thing I'd do in case of reaching the last point of the STAR, as I always thought the ATC were not expecting this and it could really mess up the traffic behind... To make things worse, the STAR chart we use most often had had the "clearance limit" disclaimer recently removed - I wouldn't know about it if I hadn't been told by someone, who has flown there before...
Call me uneducated, or whatever, but I think these procedures aer not as obvious to the pilots as the ATC would think...