A price record set to 0.00 would indicate that something is free. Whereas no price record should be treated as not for sale by way of money, and potentially act as inventory count 0 when adding to cart. Perhaps like the 'allow adding 0 inventory' option, there could be a, 'allow non priced items to be added to cart' option as well.
Making this distinction would also allow people to have more control over product purchasing when writing signals. Currently, to do so requires getting involved with the price resolution logic, since it simply returns 0.00 when there is no record.