No it dosnt make them more secure because you still only have the one seam holding the blocks togather. The two seams you sew down on each side are only sewn to the blocks. They are not holding the blocks togather. Hard to explain. You have fabric from the backing that you bring up between the two facing blocks and then you sew togather with your zipper foot, this holds the two blocks togather.
Then you fold the backing to each side of the two blocks and sew a seam on top of the blocks. This makes the sashing.
It is simple to do the Fun and Done way since you are not adding fabric to join the blocks, you are useing the backing and bringing it to the front to form the sashing.