"The wooden spacer is required to raise the cleat higher than the lip around the cabin door. The sheet can then be made to fall between the jaws of the cleat simply by pulling the sheet tight across the open cabin doorway. Without the spacer either the crew suffers skinned knuckles or a second hand is required to push rope into the cleat."
In other words, with your configuration, as you come about your crew cannot haul the line taught AND and get it in the cleat AND lean backwards over the new windward side all at once. He'll have to stay in the centre of the boat as the boat comes about fiddling with two hands to hold the sheet taught and push it into the cleat. And then later, he won't be able to release the sheet a little without leaning in the the centre of the boat, upsetting the balance, to make the adjustment and re-cleat the sheet.
With the wooden spacer, your crew can change sides and simply pull the sheet across the cabin opening (where there's no risk of skinned knuckles) at the appropriate height to ensure the line drops into the cleat and pull it tight in a single operation. You can also pull it tighter while leaning out without the risk of pulling the line out of the cleat, or pull the sheet aftwards to release it, so you can let out a little line and re-cleat all while continuing to lean back balancing the boat.
I should have added, that to make my suggestion work it looks as if the angle of the cleat needs to the changed a little so it lines up with the fairlead on the top corner of the cabin.
And Victoria is right. The one flaw with the Reedcraft design is that having the cleats in that area does stop you using the cabin as a backrest when sprawled out taking your ease mud-weighted in the middle of a Broad.