The thing is that it's only you who will know which is the file (or files) needed for the seed. It can be stored on USB sticks, or even better, on cloud, without anybody knowing what's that for. I find it a not-so-bad way to store your seed.
How is this better than just taking a picture of 12 seed words, and storing that on the cloud? It would be trivially easy to hash all hacked data to check if it returns a balance. At least a picture of 12 actual words will have some captcha qualities.
Because it's only you who will know
what those file are for (unless you store them together with wallet, unless you name them seed1.jpg...seed12.jpg), unlike storing a seed or a picture of the seed.
I mean, if I put into Dropbox a folder with 5-10 jpeg files, it's only me who will know that they're more than nice memories. (of course, as o_e_l_e_o pointed it out very good, online services tend to mess with image files, but let's ignore that for now). Unlike a backup of the seed containing the actual seed, a file is a file, really. And if you want to tell otherwise, I can tell you that I have over 3000 pictures on cloud, so if one wants to guess a seed based on (some of) those.. it won't be easy.
I find it less secure because the owner may forget the logic of his "hidden HD seed" than because one would start hashing and mixing those files to get the seed off them.