Good point! I believe that TRIM support of some form will be integrated in the near future (in file system terms - that is, several years) and that will make COW block devices more tenable. However, even with TRIM support, you run into the problem where the logical file systems are equivalent, but the same logical data is stored in different locations between the two images. The metadata is very likely to be different too at the block level.
COW block devices are a part of the solution space, they just don't completely replace unioning file systems.