I find some of XDG's default dirs, especially ~/.local/share/whatever, to be very annoying. (Almost as annoying as having ~/snap polluting my home dir, but for a different reason.) I shouldn't have to type such long paths or navigate three folders deep in order to access my data files. I therefore make use of the XDG_DATA_HOME environment variable for XDG-style programs, so they will put my files somewhere convenient. However, I don't think Snap can honor that variable, because AppArmor rules require fixed paths. Given 1 & 2, I think ~/.snap/data is a sensible compromise, at least until the underlying components are flexible enough to let the user choose.