It would help during mass production too, tho. Instead of having to make a mask and then wait x weeks for the fab to return product, you'd just buy commodity flash chips of certain sizes according to whatever storage space the producer of the game needs, then flash the game on the memory devices and stick 'em in a plastic shell... In all, process would be much faster and simpler.
I guess many of us have heard stories of how 3rd parties used to hate nintendo's draconian rules regarding carts, how they had to be ordered in minimum lots (all paid for in advance with no refunds in case game bombs), weeks in advance, with long delays in case more had to be ordered (game became a hit - perhaps unexpected hit). And so on.
Flash carts would make sense from that standpoint, but I'm still not convinced it really IS flash memory in 'em.