DHU,
spot on. Remember the rudder's sole function in life is to impart or counter sideslip. When the sideslip gets very large the forces on the vertical fin can also become very large, and this problem gets worse as IAS increases (dynamic pressure, qc = 1/2 x sea-level density x IAS^2). Many large aircraft have a sideslip limit that reduces rapidly with IAS.
So the risks of too much rudder input at speed are:
1. Exceeding the sideslip limit
2. Exceeding the structural limit of rudder and/or hinges
3. Exceeding the structural limit of the fin
4. A combination of the above.
And the problem gets worse with rudder reversals (doublets), particularly if they are applied at the natural yawing frequency of the aircraft. There was a nasty accident a few years ago where a test pilot tore the fin off a S-3 Viking by applying rapid opposite inputs of rudder; sadly the aircraft and crew were lost.
So to solve all of these problems a mechanical rudder limiter is usually used. On some aircraft it is operated manually or by flap selection, on others it is electrically actuated by a switch in the cockpit, and on most large aircraft it is commanded by a speed switch, linked to the air data computer(s).