Not directly. Much of the value in a modern FPGA lies in the specialized proprietary hardware provided by the manufacturer -- transceivers, memory controllers, clock management, dozens of other things -- and in the IP cores that can either be inferred or generated through wizards.

So knowing the bitstream format by itself is only a small step forward, if your goal is to take full advantage of the hardware and IP available. You'd need to reverse-engineer all of the specialized hardware and IP support as well. Opening the bitstream format would still be very worthwhile, but it's not the game-changer that many believe it would be.