okay so ideally to get anice bokeh to blur out the horrid backgrounds a lot of venues have so you should be shooting with a large aperture. Now day the rider is in the jumpoff so is cutting corners all over the place and you've prefocused for the center of the jump. If that rider is 1 or 2 meters away from where you've prefocused at 200/300mm you'll end up with a soft shot.
Even at f2.8 & 200mm a shift of 1 metre by the horse will not make the image soft (it is within the DofF) if you are shooting to get all the horse in the frame - I always prefocus on the centre of the jump - only exceptions are jump offs where I know corners may be cut but by then I will have already got saleable shots so I can afford to experiment.
Mike
Neil,
the OP does not have the camera lens combo that you have and therefore that is 2 reasons why pre-focus will suit them better.
Next if they know they already have a suitable focus then all they have to concentrate on is composition
and lastly they do not have your experience of timimg so whilst pre-focus may not suit you it is 4 excellent reasons to recommend that they use it