Driving with the suspension in the raised position

Assuming a tri-axle. If the trailer is run at Max height then more weight is on the front axle. If run at Min height then most weight is on the rear axle.
(Don`t believe me? Look at the tyres next time you raise or lower a heavy trailer)
So the individual axles could easily be overloaded. Tyres on that axle could be overloaded and overheat.
There would be a tendency for the lighter loaded axles to lock up under braking, although modern anti-lock should prevent this, although that would leave the heavier loaded axles doing more work. This would mean one axles brakes could get overly hot and fade.

If running under max weight on main UK roads, then it may be no real problem. Max weight on mountain roads could be another story.

As said most/all newer trailer should return to normal ride height automatically, but older ones won`t and may not have such a good antilock brake system, this will soon show on dry roads as blue smoke, as The Maoster points out, and on wet roads as a side view of your own trailer as it overtakes you! :wink: