But your forgetting that non-diabetics regulate insulin and glucose levels naturally where we have to administer insulin by injection, administering insulin by injection or pump is less predictable and for whatever reason our bodies are less effective in dealing with falling bg levels....hence why when we go hypo we have to treat with fast-acting glucose to get us out of trouble.
There has to be a margin of error with bg and that is why they state that the safe level in which insulin dependant diabetics can drive is 5mmol, were it to be 4mmol and someone's insulin is still active then in no time at all that bg could soon in up in the 2's, which undeniably is a pretty dangerous place to be.
At the end of the day these are safeguards for all diabetics and the DVLA will not make allowances by going on one persons experiences, as I say Smidge it's all about keeping road users and pedestrians safe.