The effect of this sharing of patents will not change cars overnight. It will take time, but as long as information is being shared, technology will improve faster than it would otherwise. Now, when someone comes along who wants to make hydrogen-powered cars, there won't be anything to stop them.
Toyota has not done a lot to increase the prevalence of hydrogen-powered cars. Therefore, it makes sense to share all knowledge we have available to encourage these environmentally friendly cars. Perhaps another auto maker has a way to produce these cars in a way that will encourage increased purchases, thereby reducing our oil dependence
There is no question that patent sharing by Toyota will increase production of vehicles using fuel cells. Any company with an idea that lacked the funding to do their own research will benefit greatly from this decision, and it will likely lead to new innovations and ideas in the industry. Increased innovation should lead to further production and the potential for further profits.
While I can not know for certain, I do not think the sharing of patents will increase production any time soon. I do think that it will increase research into the design and fabrication of the fuel cells, making for even more efficient fuel cells. It would be a good thing for either to happen.