We allocate products against sales order which can result in there being more stock allocated than actual stock, this is useful as it indicates what stocks we need to re-order to cover both the negative allocations and bring the free stock level back above the re-order level.
However, when using this function, you can also despatch allocated orders resulting in negative stocks and, therefore, invoices being raised for something that hasn't actually been shipped, I would prefer an option to be able to have negative allocations, for the reason stated above, but not negative stocks.