Aside from the strong moral arguments for open access - based on the fact that much of the research published in journals has been paid for by the public, who therefore have a right to see the stuff - there are also strong utilitarian grounds for making materials freely accessible.
A group at Southampton, including the irrepressible Stevan Harnad, have put together an excellent discussion of some of the amazing things that thoroughgoing open access will permit in the future.
Many of them - there are 28 in all - are positively gob-smacking, and make explicit the way in which the open access revolution will render ordinary impact factors, one of the great bugbears of academic research, obsolete by bringing in far richer metrics for measuring influence and achievement.
This is the sort of stuff that will make traditional publishers break into a cold sweat at night; but it will warm the cockles of the growing band of OA supporters because it breaks the vicious circle of "high-impact" journals being favoured by top researchers simply because they are "high-impact", not because they are the best vehicles.