Hi Simon,
Hacksworth valve gear does not produce near optimum valve events as the gear is notched up, due to it's simplification. A well designed Stephenson's and Walschaerts valve gear does provide such valve events.
When you also factor in the up and down movement of the driving axle on steam locomotives further compromising the valve events, then it is a poor substitute, except for it's simplicity and cheapness of construction.
I remember as a young Talyllyn Railway volunteer the late Ron Smith (TR Driver) adjusting the valve gear of TR No.4 'Edward Thomas' regularly. If Ron had properly understood the limitations of Hackworth valve gear it is arguable his efforts were in vain.
One of the supreme rare enjoyments of miniature locomotive driving is to have a loco with superheaters and a decent valve gear when you can open up the regulator and wind back the reverser to 20% cut off and drive on the reverser as per fullsize steam express locos! You adjust the reverser from between 25 and 15% cut off to cater for all track conditions and a heavy load of punters behind.
Very few miniature locos can be driven in this way.
Cheers,
Julian