I think the argument for MBF_DLGNOPARENT was to allow the dialog to be centered on the monitor rather than relative to the current dialog, which may be way down in the corner of the monitor. (While technically centering could be done independently of the parent, I think that as it stands, it would have to be done manually. I guess that's not really a good excuse though, is it?)
A slightly better one might that the while removing that flag would prevent the parent dialog from getting on top of the print screen dialog, it wouldn't prevent other windows, including possibly the A-Shell main window, from getting on top. We could probably debate the extent to which it made sense to allow another unrelated window to cover the print screen window, and I'm not really sure I have a firm position on that.
Another workaround would be to set the WS_EX_TOPMOST flag in the
winstylex parameter. That would prevent any window from covering the dialog, which may be perfect or overkill, depending on your perspective.